2002 volvo s40 possible gear slipping and how to fix it?

My car once in a while shifts from 2nd to 1st (automatic). I think it slips since my rpm goes up from 2.5k to 5k and then after few seconds goes back to 2.5k.

So I think that signifies transmission slippage. How to prolong the transmission life without buying a new one? The car has kbb value of $2k so to spend $1.5k on tranny does not make sense.

I need it to last me about a year before I purchase a new car.