High pressure switch stuck open how do I replace it.

You must be a trained and licensed tech to do this job

I have to agree with wmproop.

Just know this. The high limit switch may have a quick connect seat that will allow for the switch to be changed without having to reclaim the refrigerant. Or it may be the type that has to be brazed in (which would require reclaiming and a whole mess of other steps and equiptment to do the job correctly.). A trained pro will know which you have.