How to do save as on the file

I want to save it with another name after I make some major change, how do I do this?

Mac: File > Duplicate, File > Save.

iPadOS: Because iPadOS apps are not allowed to directly interact with a “file system” as such, the easiest way on iPad is probably to use the share/export icon, pick Vectornator format, and choose Save to Files.
At that stage you are in the Files app and you can rename the export file and choose where to save it.