I don't know about your BX2660, but I just changed the coolant on my BX23. On the BX23, there is a little black plastic plug on the right side of the radiator at the bottom. By "right" side, I mean right when you're sitting on the tractor. It requires a flat bladed screwdriver to loosen it.
Your 2660 may be different, and probably is since the BX23 has the D905 motor and the BX2660 has the D1005, but there is certainly a plug or valve to allow for draining. My owner's manual showed how to do it. Do you have a manual?
For my BX-25 and my Polaris ATV, I found the easiest and quickest way to drain the coolant was by disconnecting the lower radiator hose from the radiator.
For my BX-25 and my Polaris ATV, I found the easiest and quickest way to drain the coolant was by disconnecting the lower radiator hose from the radiator.