How do I change a photo extension from a tiff to jpg?

You cannot change the file format of a photo inside the Photos Library.

To change the format of a photo from TIFF to JPEG, export the photo from Photos in a different format.

• Seelct the photo in Photos.
• Then use the command "File > Export > Export 1 Photo and select JPEG or a different format for the exported file.
• Now import the exported file and delete the original file, if you want to replace the TIFF file by a smaller file.

Export it as .jpeg from Photos. Or export as original .tif and use some other app to export it as .gif (why do you want .gif?). BTW you did not say if you use macOS or iOS etc.