I have a replacement key. Can I pop out the old one or do I have to take it to a repair person?

Depends on how technical you are.
To install a new key you have to strip down the bed and slide the working keys off until you remove the broken one, and then slide everything back on again.

It's very doable but if your anyway unsure of what your doing, best to leave it to the pros. The average fee for putting in a new key is 80 Euro on my Country.

Regards
James