The Latex panel contains a "Custom" tab; up to sixteen user-defined buttons can be placed on this tab. Other buttons in the Latex panel cannot be changed, but the text inserted when the button is pressed can be changed.

The comments at the start of the file "completions.plist" explain how to make these changes. To do so, edit the file ~/Library/TeXShop/LatexPanel/completions.plist with TeXShop. Be sure to edit and save in UTF-8 format if you use Unicode characters.

How do I configure TeXShop?
Modifying the Latex Panel