I'm in need to change the iphone 6 battery, but I'm really let down by the 7-day turnaround period.

So let's say I change it at non-apple authorised service provider, do I have any concern?

My iphone 6 was bought off from machines KLCC on Feb 2015. It is 3 years old now. AFAIK, no warranty or whatsoever for a phone more than 1 year old. Correct me if I'm wrong?