Once you have accessed the AWS Turn Key Console, click on DB & Bastion EC2 Turn Key.
Click on the Change DB Instance Size link.
When scrolling down, you will come across the Instance Configuration section. Click on the select box that is currently set to,db.t2.mall, and choose the desired instance size from the options.
Scroll down all the way and click on the Continue button.
Verify the changes, and after confirming, click on the Modify DB instance button.