What is the proper way to release a DHCP lease

Hello all,

About ready to put a new box online, but need to "release" the MAC / IP address [of the old box]
if I can prior to swapping out the boxes. This might save me a call to Verizon.

I tried "dhcp release", but the OS returned a "command not found" error, essentially.

What is the proper way to get this done? I'm drawing a blank with my google fu tonight.

Thanks,

Jay