Once you have created an account you should see a link on most pages with an option to upgrade.
If your trial has expired when you login you will be directly taken to the upgrade page for the plan you selected at sign-up or the list of plans depending on what option you click on.
Settings => Plans
Once you have selected a package and completed payment your account will upgrade automatically.
If you would like to discuss anything with us before you upgrade please start a chat by clicking on the button on the bottom right of the screen.