If you wish to extend it, you must extend it before there is less than 3 months left on the warranty, so if you originally got the silver warranty (which is a years collection, parts etc), you would need to renew before you PC was 9 months old.
Yes you can extend it before it runs out, so you can upgrade to silver within the 1st month and gold within the 1st year if you already have silver etc
if you have the platinum warranty already then I don't think it can be extended further, but best check directly.