If you focus on improving your in-game decision making and build consistency in your mechanics, your off-days can only be so bad. Generally speaking better players have less variation in how well they play. So the first step to adapting to an off-day is before they happen, as you practise.
E: As far as adapting to playing poorly, there's only so much you should do - assuming you were playing correctly when you were "on", just because some decisions aren't working out doesn't necessarily mean you should stop doing them if it's the correct play a significant % of the time. Basically, don't start playing wrong/scared/bitch TF2 just because you're not hitting as many shots as you usually would / shook by the pace of the enemy team or something, this will only compound the issue.
Another thing to consider is, are you having an off-day but still winning as a team? Then you most likely don't have to change much if anything about the way you are playing that day. As some people have already mentioned 1/2 of an "off-day" is a mental issue anyway, if you let it roll off you for the next half/map/round you'll probably find yourself playing about as well as usual.