A widget .mpk is just a zip file. So change .mpk to .zip and extract the package. Or after your custom changes zip it up again and rename.
Regards,
Ronald
You can try this script as well: https://github.com/appronto/scripts
just extract the widget in the same dir, after modifying just drag the .mpk file on this script UpdateArchive_DRAG_MPK_HERE.cmd and it will repackage it for you. After that refresh your model 💪