As an American speaker, "fed up with" is more natural. I do not believe that I have heard an American use "fed up of," although it could possibly be a regional term.

"Fed up with" is the standard, traditional, and universally accepted form across all varieties of English. (e.g., "I'm fed up with the noise.") Fed up of: Common in informal British and Commonwealth English, but traditionally considered non-standard or grammatically incorrect in American English and formal writing Why does this happen? It is a blend of two similar expressions: Fed up with (standard) Sick of / Tired of (standard) So even though you hear this a lot, it's grammatically incorrect