You cannot edit individual rebars in rebar systems. There are 2 scenarios:
a. If you want to edit the rebar definition (diameter, lenght, constraints,etc), go to Properties Palette and edit the options. If options you seek are not editable in the Properties Palette (e.g. lenght, constraints), select the rebar system, got to Edit menu -> Remove Rebar System. After this you can edit the rebars as a group, change lenght, constraints, etc.
b. To edit just one single rebar, shorten your rebar system path by one rebar. Copy the rebar system, then Remove Rebar System on the copy. Delete all copied rebars except one single rebar. Place the single rebar in its' place, then edit it as you want.