Can I update multiple forms (50) simultaneously?, they are similar at 98%. or maybe importing a source code to update existing form?

Please Help me

I'm afraid that's something that can't be done. You'd have to update each form separately even if they are 98% similar with each other.

The closest thing we have that involves multiple forms is managing fields in between them (e.g. cloning a field or fields from 1 form to another). If you're interested, here's the link to the app https://apps.jotform.com/app/field_manager