Learn Dutch: Heb jij je bedacht? - Have you changed your mind?

This uses the reflexive verb zich bedenken. The first word, jij, is the subject ('you'). The second word, je, is the reflexive pronoun ('yourself'). In Dutch, you literally ask 'Have you yourself changed-mind?'.

Changing your mind sounds like a change of state. Why do we use hebben instead of zijn here?

You are right that verbs describing a change of state often use zijn in the present perfect. However, there is a strict rule in Dutch: all reflexive verbs always use hebben. Because this verb has the reflexive pronoun je, it must use heb.

Why doesn't the past participle have a ge- at the beginning, like gebedacht?

The infinitive of the verb is bedenken. Because it already starts with the inseparable prefix be-, we do not add the standard ge- prefix to make the past participle. It just becomes bedacht.

I thought denken meant 'to think'. How does this mean 'changed your mind'?

Adding prefixes and reflexive pronouns changes the meaning of verbs. The base verb denken is 'to think'. Adding be- makes bedenken, which means 'to invent' or 'to come up with'. But when you make it reflexive (zich bedenken), the meaning changes completely to 'to change one's mind'.