The easiest way is simply to uninstall and unpublish the ISV solution. Then you are back to a vanilla BC.
But that will mean that you lose the data that i stored in tables and table extensions used by the ISV solution.
If you want to save those data you have to export those data out of Business Central before you uninstall and unpublish the ISV solution.
For exporting them you can export to excel, use xml ports or configurations packages.
Test in in a sandbox environment before you do it in production.
This prosess is basically the same process that will happen if you purchase and ISV solution and for some reason the ISV provider stops to maintain the combability with the new versions of BC online.
You will never lose data stored in standard field in standard tables using this methode.