The thumbnails are small low resolution version of your image intended for the thunbnail view. You can't convert them back to full resolution images.
What we need to understand is why you now have thm files rather than the the proper versions. Please reply with details of your system, eg:
MacOS version
Where is your library stored
Do you use iCloud photos - and if so, have you set "optimise mac storage"
Did anything happen prior to this happening (eg system upgrade, or move of the library.
How big is your library.
Are you sure you don't have both versions of the photos - a jpg and thm somewhere in the all photos view?
Also please post a screenshot of what you are seeing.