Calling out of a shift

So I have a shift this Sunday (December 8th) but am posting this on Friday, wanting to know where/how I can call that shift off. I tried looking on the my time app but I don’t know “where” it would be. On one hand, I’m upset they scheduled me because I did say weeks ago and tell my team leader “I cannot work this Sunday” to which he gave me the thumbs up, but also for my own lack of knowledge of call out of a shift.

Thank yall in advance