Please refer to the FAQ for Azure for Students. As it states in the first question, once the year is up you can renew again provided you are still a student. This starts another year of the student subscription.
If you have used all your credit then you cannot simply renew to get more. It is one credit per educational account. You can switch to Pay As You Go if you want to continue using services that cost credits.
Subscriptions, like pretty much everybody else, provide a fixed set of credit. If you don't use all your credits before the subscription expires then they are lost. They do not "roll into" new subscriptions. So, for example, if you had $25 left on your old subscription and then renew for another year that $25 credit is gone and you start over again with $100 credit for the subscription> It is a "use or lose" system.