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.
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 💪