can i change script api version after creation?

You can upload the script with a new API version and the UI should reflect the update. Your other option (or was an option) is you can direct which version of SS should be run if the script specifies SS

2.X

. We consider it a best practice to be explicit about SS versions in our script headers so we don't really use

2.X

when i change the script to different version, it doesnt let me save it

if you are going from 2.1 to 2.0, you will need to remove features that use syntax from ECMAScript 6

also, I think the 'online code police' that runs when you save in the UI is different between SS 2.0 and 2.1