Mac mini stuck on update

I am about one week into an update to my mac mini. They machine has worked perfectly since I've owned it (3 years) and have it backing up daily, however I haven't keep the OS updated regularly.

I launched an update last week and it's been stuck about 1/3 of the way through, often times with "estimating time remaining" messaging provided. I've turned it off and turned it on several times, and done a disk utility "first aid" to all the disks with no issues found. My next option appears to be restore the disk from a previous backup - but considering this machine (and presumably my backup) has a lifetimes worth of photos, I'm hesitating to click the button to wipe the existing disk.

Can I just undo the update and start over?