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Understand the conditions under which you can do this. Check to see if the Excel file is encrypted. Make a copy of the protected sheet. Enable file extensions. Change the Excel file into a ZIP folder. Extract the ZIP folder. Open the "xl" folder. Open the "worksheets" folder. Open the sheet in a text editor. Remove the password protection code. Save your changes and close the text editor. Copy the "worksheets" folder. Open the ZIP folder. Replace the ZIP folder's "worksheets" folder with your copied one. Change the ZIP folder back into an Excel file. Open your Excel sheet.
If only the Excel sheet is protected—that is, if you can open the Excel file and view its contents but not edit it—then you can use this method to remove the password protection on both Windows and Mac computers. If the Excel file itself is encrypted, you cannot use this method to remove the password. The easiest way to do this is by double-clicking the Excel file; if the file opens like usual when you double-click it, the sheet is protected but the file is not.  You should see a pop-up warning appear if you attempt to edit the Excel sheet. If you're immediately prompted for a password upon double-clicking the file, the file is encrypted and you cannot use this method to open it—try the next method. Click the Excel file that contains the sheet you want to unprotect, then press Ctrl+C (Windows) or ⌘ Command+C (Mac) and paste it elsewhere by pressing Ctrl+V (Windows) or ⌘ Command+V (Mac). This is necessary in case you accidentally corrupt the original version of the file in the process. Skip this step on a Mac. If you're using a Windows computer, you'll need to make sure you can view and change file extension names by doing the following:  Open {"smallUrl":"https:\/\/www.wikihow.com\/images\/b\/b1\/WindowsFileExplorer.png","bigUrl":"\/images\/thumb\/b\/b1\/WindowsFileExplorer.png\/30px-WindowsFileExplorer.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Windows icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} File Explorer (or press ⊞ Win+E). Click View  Check the "File name extensions" box. To do so:   Windows — Right-click the Excel file, click Rename, delete the "xlsx" text at the end of the file's name, and type in zip. Make sure that you keep the period between the file's name and "zip". Press ↵ Enter, then click Yes when prompted.  Mac — Click the Excel file, click File, click Get Info, delete the "xlsx" text at the end of the file's name, and type in zip. Make sure that you keep the period between the file's name and "zip". Press ⏎ Return, then click Use .zip when prompted. This process will vary depending on your computer's operating system:   Windows — Right-click the ZIP folder, click Extract All... in the drop-down menu, and click Extract when prompted. The extracted folder should open.  Mac — Double-click the ZIP folder, then wait for the extracted folder to open. Double-click this folder in the extracted folder to do so. If the extracted folder didn't open for some reason, first double-click the regular folder with the same name as your ZIP folder. It's near the top of the "xl" folder. Depending on your computer's operating system, do one of the following:   Windows — Right-click the sheet you want to unlock (e.g., "Sheet1"), select Open with in the drop-down menu, and click Notepad in the resulting pop-out menu.  Mac — Click the sheet you want to unlock (e.g., "Sheet1"), click File, select Open With, and click TextEdit. Find the "sheetProtection" section that's inside the "< >" brackets, then delete everything from "<sheetProtection" to the closed bracket ("/>") on the other side of the sheet protection algorithm. Press either Ctrl+S (Windows) or ⌘ Command+S (Mac), then click the X (or red circle on a Mac) in the corner of the text editor. Click the "Back" button to go back to the "xl" folder, then click the "worksheets" folder and press either Ctrl+C (Windows) or ⌘ Command+C (Mac). Double-click the ZIP folder that you created earlier. Navigate to the ZIP folder's "worksheets" location by double-clicking the "xl" folder, then delete the "worksheets" folder, click a blank space in the current folder, and press either Ctrl+V (Windows) or ⌘ Command+V (Mac). This will paste the copied "worksheets" folder into the ZIP folder. Close the ZIP folder, then do one of the following:   Windows — Right-click the ZIP folder, click Rename, replace the "zip" text with "xlsx", and press ↵ Enter. Click Yes when prompted.  Mac — Click the ZIP folder, click File, click Get Info, replace the "zip" text in the title with "xlsx", and press ⏎ Return. Click Use .xlsx when prompted. Double-click the Excel sheet, then edit it as needed. If you receive an error that the Excel sheet is damaged, you probably removed extra code when you were attempting to remove the password protection algorithm. Repeat the above steps, making sure to only remove the text between the brackets (</>) and the brackets themselves.