go to KSP\GameData\Bluedog_DB\Parts\Apollo
find the engine you want.
It will be bluedog_Apollo_Block5_ServiceEngine.cfg the 5 will change to 2/3 for the other two
right click open with notepad++ find the line that says maxThrust and change that value to what you want.
the line will look like this:
maxThrust = 33
generic notepad will do, i also think that if you go in to KSP and press alt+12 then go to database and reload you can the parts database without reloading KSP, this does provide some isues with TweakScale though