On my furnace I'm getting 3 red lights, indicating pressure switch stuck open. How do I clear this please?

Without such info as the make or model, this video series provides some helpful information.

https://www.youtube.com/watch?v=NLfb1Il_YB0

You don't clear, you determine if the switch is dirty and not allowing closure or is bad. There are many different switches. Google " testing a... inducer pressure switch.(put your brand in the... )