You can back a project in two ways:
a) Back Button: Click on back button and you will be re-directed to the page with all the list of rewards and an option to back any amount to the project.
c) Reward Selection: choose the reward listed on the project profile page and you will be redirected to the page where that reward is pre-selected.

Next step is to fill your delivery address and pay for the back amount using available payment options.