No biggie, just insert the U shaped tool or the two pins that you used to get it off the first time and it will come right off.
When you put the new lock cylinder in, turn it to what ever position the old one was in and slide it into place without the collar. If you're not sure what position it needs to be in, Turn the switch in the hole that the lock cylinder goes in, all the way to the off position (counter clockwise) then put the lock cylinder in without the key in it.
Got to go, they already kicked off!