Bicycle Tutor

At the moment I have a compact gear set with a 50-34 chain ring on the front, if I want to change this for a 52 -39 on the front will I have to change the company derellier on the back, and if so why

If it was the other way round, you'd run into problems. The limiting parameter is the so-called "capacity" of the derailleur, that is, how many teeth difference it can accommodate (the additional length of the chain, difference between "small - small" and "large - large" gear combination). This is why derailleurs come in different sizes (cage length) for road bikes, depending on the gear combinations one wishes to install. Since the needed "capacity" is smaller after your modification, you do not have to install a new derailleur. However: shifting performance could benefit from a shorter cage.
All the best,
Joe
EDIT:
Also, shifting performance will most probably benefit from a new front derailleur.

Thanks for that I will try it this weekend
Mike