How can I change a password on a dmg?

What I ended up doing was creating a new .dmg then dropping/pasting the applicable files into it. I found it to be more straightforward and controllable than using Disk Utility. YMMV.

Although in the Disk Utility help article it is explained that a password can be changed at: Disk Utility > select .dmg in the sidebar > File > Change Password :I always found/find Change Password not selectable. Perhaps I misunderstood/stand the help article:

Disk Utility for macOS Sierra: Protect a disk with a password using Disk Utility

Another option is to Convert. I did try this in a practice .dmg with dummy files and it did work. In the Convert process, after you choose an encryption option you're prompted to set a password.

Convert explained here:

Convert a disk image using Disk Utility on Mac - Apple Support

You’re welcome.🙂

With a solid backup plan (think multiples in method and destination) in place, there’s no worries as to the how of accomplishing the task. 😉