Installing a blender plugin is as simple as it can be. After you have made a purchase, either on blendermarket or gumroad. You’ll have access to download the zip file of the plugin
Open Blender and go to Edit>Preferences
Go to Add-ons
Click on the tiny downwards arrow on the top right corner and select Install from Disk…
Just browse and find the zip file downloaded from the website and click Install From Disk
Open Blender and go to Edit>Preferences
Go to Add-ons
Type “stitch3r” in the search box to find the addon
Then just click the “uninstall” button and you are done.
You will have to uninstall the old version as shown above and install the new one. You can do them out of order, installation of the new one, and then the uninstallation of the old one but that will require you to restart Blender when done.