If you check out a file and don’t make changes to it, or you make changes that you don’t want to keep, you can simply discard the checkout, rather than saving the file. 

• Go to the library where your file is saved.
• Select the file, or files, that you want to discard changes for.

• Above the Documents list, select the three-dot menu, and then select Discard check out.

Click OK to finish discarding the checkout. The red "checked out" icon disappears when the file is checked back in.