There are different ways you can achieve this. Some of which are
1) You can write a SQL Server Job and schedule it to run daily to check users whose subscriptions has expired and auto-renew it. But you will require permissions to do this in your Database. Contact your DBA for the permissions. Check this link to learn on how to schedule a Job.
2) Write a console application which will interact with the Database and do the similar job. You can schedule your exe using the Task Scheduler in Windows. The Link below explains how to create a scheduled taks in Windows 7.