You can extend the registration time of your .ETH name at any moment and you can pay in advance for as many years as you'd like. The fees for renewals are set by the ENS Dao and are the same as the initial registration cost based on the length of the name: 5 characters or longer cost $5/year, 4 character names cost $160/year, and 3 character names cost $640/year.
You can renew names without being logged into the Ethereum account that own the name since any Ethereum wallet can pay the renewal fees for any name (without affecting who owns the name).
The ability for any Ethereum account to renew any name is a function that is only there for your convenience. Paying to renew a name that does not belong to you does not give you ownership of the name nor does it make it such that you will receive the name if it expires.