Summarize this article:

Click or double-click the Chrome app icon, which resembles a red, yellow, green, and blue sphere. It's in the top-right corner of the window. A drop-down menu will appear. This is near the bottom of the drop-down menu. Selecting it prompts a pop-out menu. You'll find this in the pop-out menu. If Chrome asks you to update, click Update Google Chrome and wait for the update to install. If Google Chrome is up to date, skip this step and the next one. Once the update finishes installing, you'll see this button. Clicking it closes and re-opens Chrome. Go back to your video and click the "full-screen" icon in the bottom-right corner of the player window. Your video should now play in full-screen mode. If your video still isn't playing in full-screen mode, try one of the remaining steps in this method. Restoring default settings may fix the full-screen problem, though keep in mind it will also remove your current settings:  Click ⋮ in the top-right corner. Click Settings. Scroll down and click Advanced. Scroll down and click Restore settings to their original defaults. Click Reset settings when prompted. and then reinstall it. This will help you force a Chrome update if you know one is available but Chrome isn't updating. You can reinstall Chrome by going to https://www.google.com/chrome/, clicking Download Chrome, clicking Accept and Install, double-clicking the downloaded setup file, and clicking through the on-screen prompts.

Summary:
Open  Google Chrome. Click ⋮. Select Help. Click About Google Chrome. Allow the update to install if prompted. Click Relaunch when possible. Try using YouTube in full-screen mode. Reset Chrome to default settings. Uninstall Google Chrome