Is there a trick to replacing the extractor in a 10/22?  Are there instructions available online?

Use a small punch to force the spring plunger back, and shake the extractor out. Slowly release pressure on the plunger and spring. Reassemble in reverse order. Replacing it is even easier. It's just that simple.

Some people like to do this with their hands and the bolt inside a clear plastic bag in case they lose control of the thing so the spring and plunger don't go flying across the room.

The plunger has to pushed back into the bolt to give the clearance needed to lift the extractor out.  As  Snake45 mentioned a small punch would work as well as something like a dental pick.  rimfirecentral.com is a great place to look up stuff like this.  

+1 for rimfirecentral.com