Mac Mini connecting to Airport Extreme at only 802.11a, how to push to 802.11b/g/n?

Mac Mini connecting to Airport Extreme at only 802.11a, how to push to 802.11b/g/n?

My Mac Mini is capable of 802.11a/b/g. I have an Airport Extreme that is capable of 802.11a/b/g/n. The Mini connects at only 802.11a. It is close and has a very strong signal. How do I push the Mini to reconnect at 802.11g?