You can edit the rebar cover of either the entire element or a particular face while working with rebar and its host.
 Reinforcement panel
Reinforcement panel
 (Cover).
 (Cover). 
		   (Pick Elements) to pick an entire element or click
 (Pick Elements) to pick an entire element or click 
			  (Pick Faces) to pick a singular face of an element.
 (Pick Faces) to pick a singular face of an element. 
		   on the Options Bar or Structure tab
 on the Options Bar or Structure tab Reinforcement panel drop-down
Reinforcement panel drop-down
 (Rebar Cover Settings).
 (Rebar Cover Settings).