Where to save changes to Attach Invoice

Good day
I have a custom invoice I use for the attach invoice plug in.
Every time I change servers (we've had issues) or reinstall etc I override those changes.
Where is the best place to save the custom invoice so that it is not affected by upgrades and reinstalls.
Thank you

You can edit the file plugins/hikashop/attachinvoice/attachinvoice/invoice.php directly.

There is no need to reinstall the attach invoice plugin each time you update HikaShop. Once you have it installed, that's it.
Upgrades and reinstalls of HikaShop won't affect that plugin files nor your modifications in that file.