It's under "Settings" right below "About" and reads "Update To Pro- Never see ads again". UNLESS...
How long have you had Textra? I had this same issue. When I first got the app I was instantly head over heals and sold in the first hour so I had no need for the trial period, I wanted to upgrade right away. But I couldn't find the upgrade option anywhere! Finally gave up. I really can't remember exactly how long but I think it was about a month later I received a text from the Textra Team informing me that my ad-free trial period was up and if I'd like to continue using Textra ad-free I'd need to upgrade to Pro for $2.69. It then gave me the option to do it right then and there or wait, at which point the Upgrade To Pro option was added to the settings panel. If you've had Textra less than a month this is why you can't find it. But if you've had the app over two months and still can't find the option under settings...I'm not sure. Maybe you're sol. Try emailing/contacting the Textra Team. Good luck dude. Hope this helps