That looks like it's only 5 speed? Were there 5 speed UG's? If it's not a UG, it's a freewheel. You need the right freewheel tool to remove it - a bike coop is likely to have it.
But then you need to replace it with a freewheel, not a cassette, so your plans may need to change.
To help us better identify it, get a closeup photo of the freewheel body, after you've removed the wheel and quick-release from it.
EDIT: Yeah, what garage sale GT said.