I have an active Schengen visa which expires in June this year. However, I will be traveling to Greece at the end of July and will need to renew my visa. My question is- by when do I need to apply for a new Schengen visa? Can I do it while the existing visa is still active or do I need to cancel it and apply for a new one OR Do i have to wait for it to expire before I can apply again?
The usual rule applies, i.e. you should apply about three months before the trip. If your visa is still valid but will expire before your next trip it's not a problem, your new visa will simply not be valid immediately but only for the dates of your trip. The only thing that consulate want to avoid is overlapping periods of validity.
For multiple-entry visas (apparently your case?), you can even apply earlier (up to six months in advance) and should get a visa with a period of validity starting right after that of your current visa. With visas like that, it's even possible to plan a trip spanning two visas (entering on one visa, leaving on another) as long as there is no gap between them (and of course you don't stay too long in the Schengen area).