I think that what you posted at the bottom of your post looks like a C# function? I don't think that is what your professor is looking for. They wants the entire logic inside your proc. I agree 100% with Luis that MERGE would be the best approach here.
Mostly you seem to be on the right track although I don't understand the trigger part at all. Also be careful, you have referenced EndDate in your query but that column is not in your Users table.
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