In one sentence, describe what the following article is about:

You can access this website on a computer, phone, or tablet. Even if you initially booked your flight through a travel company (such as Expedia or Orbitz), you can still find your flight information on the airline's website. Several options will appear. Select this option if you want to look up your reservation and/or make modifications.  If it's fewer than 24 hours from takeoff time and you want to check in for your flight, click Check-in in the menu, enter your flight details, and then click the yellow Confirm button. If you just want to check the estimated arrival and/or departure time for a particular flight number, click the Timetable & flight status link, select a lookup method, and then follow the on-screen instructions. If you have a Lufthansa online account, click Lufthansa ID, enter your login details, and then click Login to continue. If you don't have a Lufthansa account, you can log in using your Booking Code (it should be in the confirmation email from your ticket purchase) or using your Miles & More number and PIN. If you sign in with your Booking Code, you'll be prompted to create an account. Follow the on-screen instructions to do so when prompted. If you have multiple flights associated with your account, choose the one you want to view and/or modify. This displays all relevant information about your flight, including your preferences, upgrades, and other details. Some options that may be available are reserving a specific seating position, upgrade your seat, and/or updating your meal preferences.

Summary:
Go to https://www.lufthansa.com. Click the menu ☰ at the top-right corner. Click View & amend flight details. Select a sign-in method. Select the flight you want to view. Follow the on-screen instructions to modify your registration.