I want to change the po of the purchase order after generating it i just want to add a prefix before it what kind of script can i write to do that

You can setup prefixes for the generated PO in Setup>Company>Autogenerated numbers. Or are you talking about adding prefixes to existing POs?

Also have a look at 2021.1 auto numbering rules.

Yes adding prefix to existing pos depending on a value from purchase order screen