WARNING: This page is about a different car, the 2021 Audi S4 and 2021 Audi A4 Quattro. However, it is still accessible from the selected car via links, so may be relevant.
-- Pry off the cap -3- with a small screwdriver.
Fig 1: Footrest
-- Remove the bolt -2- behind it.
-- Pull the footrest -1- downward in direction of -arrow- and remove it.