How do I use the kde-unstable repo on Arch?

As long as you still have access to the cli it should be fixable. If you want to still try to get to plasma 6 make sure you also enabled the core-testing and extra-testing repos in addition to kde-unstable as per the wiki

If you enable any other testing repository listed in the following subsections, you must also enable both core-testing and extra-testing

I missed that little snippet when I first swapped over.

If you do yay kf6 you can install all of the framework-related packages which might also help fill out some missing dependencies. For me it's 1-71. You can do the same with yay plasma and then choose the ones from kde-unstable (122-194 for me) but you will have to manually avoid the ones with conflicts like plasma-framework.

But if you want to try and revert theoretically simply removing the testing and unstable repos and doing another sudo pacman -Syu should get you back onto the older versions.

Hopefully helpful:

you need the following packages for kde-unstable:

• plasma-activities
• plasma-activities-stats
• libplasma

Your Update likely removed these from your system breaking plasma6

The following packages are likely to blame (this list is likely incomplete):

• kweather (I removed it since I don't use it)
• dolphin (as suggested above switching to dolphin-git should help, alternatively installing extra/dolphin will install the plasma 5 version bypassing the dependency problems)
• kio-extras (same fix as for dolphin)
• sweeper (see above)

Edit: if you have terminal access you can try the following (no guarantee it'll work, I think it should but no guarantees):

• pacman -Rdd kactivites kactivites-stats plasma-framework
• pacman -Syudd libplasma plasma-activities plasma-activities-stats

What I did (switch to plasma 5 versions):

• pacman -Syudd extra/dolphin extra/sweeper extra/kio-extras extra/kweather

What you could do (switch to git):

• $AUR_HELPER -Syu dolphin-git sweeper-git kio-extras-git kweather-git