My dad has been diagnosed with stage 4 pancreatic cancer and is terminally ill and I am on probation in California but came out here to be with my dad in Arizona , how do I write a letter to a judge asking for early termination of supervision something that I feel I shouldn't be on in the first place?

These are the circumstances that a judge may consider.

https://www.shouselaw.com/early-probation-termination.html

There were many templates to guide how you construct a letter to a judge on the internet, and I would also include a letter from your dad's doctor describing his situation to document the reason for your request. Be aware though that much depends on what you are on probation for. Just to ask did you clear your visit to Arizona with your probation officer? If not, then a letter to the judge would be pointless.

Good luck to you and your dad.