I have some BTC from grid mining I did 6 yrs ago...all I have now is the bitcoin address. How can I spend it? I've changed computers at least 4 or 5 times since then, and didn't keep much SW around when I changed.

I've confirmed that it's NOT SPENT (at least at blockchain.info), but I don't even remember what program I was running to mine it, let alone if I needed some wallet to keep it in (or if I HAD a wallet, it's long gone...)...at the time, all I thought I needed was the bitcoin address (starts with 13Ug6j...).

How can I get it into a wallet (or someplace else) so I can spend it or convert to USD?
You can't. An address is not enough to spend coins. You need the private key that corresponds to that address; if you don't have that, then you can't spend the coins. You cannot get the private key that corresponds to an address by only knowing the address; rather you must have had that private key since it was generated and never lost it since then. If you do not have the private key, your coins are lost and unrecoverable.