How to go about maintaining individual app installs on websites.

I have a niche set of clients and am about to complete a small app that they run on their website.

It has a database and codebase for back and front end.

Let’s say I get this app onto 20 client machines and then need to update a couple of php files a js file and add a column to a table.

Do I go to each website and update it or is there a way to manage updates from a central location?