You can extend it by recharging with a voucher. If you recharge your account by an amount less than Rs. 100, it'll increase by 15 days. Check their site for the actual validity limits.
Oh - and they're not like Ufone where the recharge date isn't "extended", but merely a certain number of days ahead of today. For instance, if today is 1st Jan 2009:
Telenor - a recharge will mean an extension to your already existing expiry date - even if it's set to expire in 2011 (suppose your current expiry is 20 June 2011, and the recharge voucher has a limit of 15 days, your new expiry will be 5th July 2011)
Ufone - it'll only extend by a certain number of days from Jan 1 (e.g. Jan 15 2009 if the validity is 15 days)
A little complicating, but it's easy. I hope this helps.