As @DigitalNinja says, solder and crazy glue would do it. Perhaps get a hand lens, or one of those lighted magnifier strap-ons. so you can see close up.
It looks like the trace leading to the missing pad is going through the white letters "R6", then dives into the board through a small round "via." Use the back of a razor-knife to scrap all the green soldermask off this trace and via. Don't scrape off the copper.
I'd try first gluing down the torn-off pad in its original place. (Carefully unsolder it from the switch, without bending it, then glue it down using a tiny droplet of crazy glue.)
Then, use solder to "tin" the scraped copper trace, then solder a tiny wire from the glued pad to the trace. Finally, solder in the new switch.
Normally it's a very bad idea to glue down a component, but for a heavily-used button you might consider it. Crazy-glue under the switch body after it's soldered, since the copper pads are the only thing holding it down, and they aren't as strong as before. (Otherwise, glue parts down with silicone caulk, which doesn't prevent later removal.)