Or, View->Add-Ins, select the add-in, click on the folder link. That is the install folder for the add-in, so any changes you make there will be available the next time the add-in loads. (And the changes will be lost if you reinstall the original add-in.)
@frank_wang solution is also good if you want to unzip, change, re-zip, reinstall. Renaming the add-in (so it installs in a different directory, perhaps with a different name on the menu) requires small edits in addin.def and addin.jmpcust files.
View->Add-ins can enable/disable or remove old add-ins
Craige