If the file is being edited by another user, then you cannot check-out the file to make any changes. Request the user to check-in the controlled file so that you can then open the file for edit.
This option will send an email message to the user that has this file checked out. The automatic email created will be addressed to that user, and request that they check-in the file so that you can make changes. This option is only when the file is checked out to another user.
Request a check-in of a file by:
Go to: Documents tab.
Navigate to the folder that contains the file that you want to edit. If it is being edited by another user the checked out symbol is shown against the row.
Select the row action button and select the 'request check-in ' option.
An email message is automatically sent to the user that has this file checked out. That email will ask the user to check-in the file so you can make your required changes.
Check back in the document library later on to see if the user has now finished their work on the file.