Generally, in most car seats, to adjust the straps you need to unhook them from underneath, take the straps out and put them in the next row of holes. This means that yes, you will have to take the carseat out to do it so you can get to the straps.

I dont know what your carseat is like but this is what all my carseats have been like so far. I suggest googling your carseat instructions or looking it up on a certain "tube".

Hope you get it figured out