To upgrade your plan, you will need to go through the data-import and review processes again.
Click on Dashboard menu item, then click "Import More Transactions" at the bottom of your screen.
Click "Finish Importing Transactions" orange button on the bottom right.
Then go through and make any adjustments you need.
When you get to the Reports page, you will see that you have the option to upgrade again. This is where you will be prompted to pay for the higher level plan.