I found the solution:
On my iPhone go to
Settings
At the top touch my name
Touch Subscriptions
That shows all of my subscriptions and I was able to cancel my donation there.

I agreed to donate $2.99/month but now would like to subscribe to the premium version instead. However I cannot find a way to make that change and cannot find a way to cancel that donation. It appears that once you agree to that you cannot stop it. Can anyone help with this?