To change the good cause you support, go to this page: Change the cause I support.
Next, search for the cause that you'd like to support, select them from the search results, and then press "Switch to this cause".
Going forwards, any donations raised will go to your newly selected cause.
Other ways to phrase this question
How do I change my cause
I would like to change the school I support
I would like to change the cause I support
How to change my nominated organisation
How do I change who my donations go to
How do I swap causes