That's not currently supported, but you can try copying the model files over top the existing ones.

Your subscribed Steam workshop files should be cached under a folder like this:
C:\Program Files (x86)\Steam\steamapps\workshop\content\1049410

Find the folder that has your model, try copying your new model files over top the old - just make sure that your new version of the model file keeps the same filename. Then in the game, you might need to grab your model and update the title or description (or thumbnail) and to make it "dirty" so it will let you update it to the workshop.

The one thing I'm not sure about is that the Steam client might overwrite the workshop files if it detects they're different? So you might need to copy over them while the game is running or something.

Ah, Okay. Thanks for the work around. I'll give it a try.

Update: That worked! I just replaced the files in the folder and then updated the photo in the workshop level and was able to update the Steam Workshop object. Might want to add that to the guide.

Where are the "Personal Collection" models stored? I try to delete them, but they just keep repopulating a bunch of duplicates every time I load the workshop level even after deleting them all. I want to see if deleting the files from the folder works, but have no idea where to find them.

EDIT: Found them! Deleting all the files in there fixed the problem.
(C:\Users\"USER"\AppData\LocalLow\PillowCastle\SuperliminalSteam\UGC\Models)