Changing O2 sensor on 2000 Toyota Corolla

How do I get the harness to release? Thanks.

Disconnecting electrical connectors can be frustrating. Some connectors have squeeze tabs. Some have tabs to lift. Some times, on some connector, I have lifted the tab(s) with one screwdriver (or, two for two tabs) and used another screwdriver to encourage the halves to part.
Your repair manual may have a picture to show you how. Does it?

This could help: http://www.autozone.com/N,16701151/shopping/partTypeResultSet.htm