Hoping I understand what you are asking here:-

True fully if you are new to biking & do not have any tools & very limited knowledge, no choice take it to your local bike shop explain the situation, basically you cannot change crank arm length without changing the whole crank out i.e. left & right side, to your new desired length, & make sure you end up the same crank teeth 34/50.

I cannot see any reason why you need to change out the BB ? others on the forum may know more detail.