Sharepoint - Check out, check in, or discard changes to files in a library

If you want to make changes to a file on a site and you want to make sure no one else can edit it, check out the file. When you have the file checked out, you can edit it online or offline, and save it—multiple times, if necessary.

When you finish editing and check the file back into the library, other people can see your changes and edit the file, if they have permission. And, if you decide not to make or keep any changes in the file, you can simply discard your checkout so you don’t affect version history.

When you want to check out and edit a document, whether it's working online, or downloading, do it in the following sequence:

Check out the document
Open the document in a supported app or download
Edit the document
Save the document or upload
Check in the document

Check out files

Some libraries are set up to require checkout of files. If checkout is required, you will be prompted to check out any files that you want to edit. When you are finished with the file, you need to check it back in or discard the checkout.

If checkout isn’t required in the library, you don’t need to check it out as long as you don’t mind letting other people edit at the same time as you.

Go to the library where your file is saved. If you’re looking at a view of the library on another page, you may have to click the title of the library first. For example, you may have to click Documents first to go to the Documents library.
Select the file, or files, that you want to check out, and then select Check out.

Check in files

Your files and any changes you make to them, will not be available to your colleagues until you check the files back into your library

Go to the library where your file is saved. If you’re looking at a view of the library on another page, you may have to click the title of the library first. For example, you may have to click Documents first to go to the Documents library.
Select the file, or files, that you want to check in, then select Check in.
In the Comments area, add a comment that describes the changes you made. This step is optional but recommended as a best practice. Check-in comments are especially helpful when several people work on a file. Moreover, if versions are being tracked in your organization, the comment becomes part of the version history, which may be important to you in the future, if you need to restore to an earlier version of the file.
Click OK. The green arrow disappears from the file icon when the file is checked back in.