You can view in-app purchase options under MORE > ACCOUNTS. If your current plan is not listed as an in-app renewal option, then you will have to wait until your subscription is expired to see a purchase option for your plan (in some cases, your plan may be presented 2 weeks prior to expiration).
If you want to use an iTunes gift card towards a renewal, be sure to redeem that card into your iTunes account BEFORE you attempt your in-app renewal. For more details, see this article.
If you wish to renew your subscription early, you can do so through our website purchase page. If your plan is not listed on that page, consider using our Build Your Own Plan option found at the bottom of the page.