Q: Appointments - If you're not the organiser but just a participant, can you remove yourself from an appointment that another user has made in Upsales? – Upsales Help Centre

A: If the organizer is another Upsales user you cannot edit the meeting
If the organizer is an external host you probably can remove yourself in Upsales, but since the meeting is still in your calendar in O365/Google I think you will be re-added to the meeting again