You can renew at any time (which will add on to your current subscription) by logging on to http://members.iracing.com.
Then go to Account. Log into your Account.
Then click the Renew My Subscription button.
In the Subscriptions Choices section, choose the desired length from the drop down.
Then click on the Renew Early button at the bottom and proceed with your purchase.
This will add the time onto the end of your current membership and you will not lose any time.
For example, if your account is set to expire in March, but you want to renew early in January for another year, it will add a year onto the end of your membership making your new expiration date March of next year, not January of next year.
If you purchased iRacing through Steam, you will need to contact Steam Support for further assistance.