@all,I have a Client's SCA Netsuite account with so many active extensions. Could someone let me know what is the best way to update/change into an existing extension? Note that I don't have any code repository where they maintain code. Please someone let me know the few steps?

Download extension development tool, fetch extensions. It will let you fetch extension if those are not locked ones. Otherwise you'll not be able to get the source code of those.