I had an assembly in a folder. I copied the folder, renamed it, and started making changes. Now, when I want to save a modified part, I can't. I don't have the "save" option. The document was already saved in my old file.
How do I save the part in my new folder (it's a completely different part )?
The save icon is not normally in the drop-down menu you show but I may have misunderstood the pb.
I suggest you start by changing the name of each piece or set that will be changed.
Then check the paths of the parts and correct them if necessary before opening the assembly. When selecting the assembly you want to open, click References in the lower-right corner of the opening window.
This will open another window that will show the paths of all the parts in the assembly.
If they are not correct, edit them by double-clicking on each path and navigating to the new location.