If you're going to try I assume that the Cessna's 4 channel with ailerons (if it's 3 channel I REALLY wouldn't even think about trying the Yak before an aileron trainer) then slowly not all at once and not both at the same time, increase the throws and move the CoG back and it'll liven up it's responsiveness and sharpen up your responses.
Just to give you a comparison before I went to the Acromaster which can be set up to be more docile than the Yak I'd flown a MiniMag for 5 months and had got to the stage that I was literally throwing the plane around in the sky knowing that if I got it totally wrong I could correct it rather than crash and was flying full circuits inverted.
If you launch it and it's not trimmed right use the sticks to fly it to almost double teh height you normally fly at bring it back to 40% throttle with that motor (or less) and then think about trying to trim it .
Personally I'd hang onto it and keep it as a good 3rd / 4th plane but if you insist on flying it go to a club and get some help !!
I know you got it for nothing but it would still seem a shame to wreck and then go and buy something you can fly because if you do that 1st then you've already got something for after that and would suggest you get something that would use the same motor, esc, batteries etc so your only cost is the airframe and then transfer the guts later !!!