Your purchased download cannot be changed by opening it through 'My Stuff'. Instead, the design needs to be updated, and then generated again as a high-resolution download.
To make the changes, go to 'Order History', and click 'Make a Correction', beside the download button. You'll be redirected to the editor and prompted to make changes. Once you've finalized the changes, click 'Save and Recreate Order'.
The high-resolution download will generate and will be ready in 5-10 minutes. You'll receive the new download from the Download page.
Keep in mind these changes can be made 3 times. On exceeding this, you will be prompted to purchase the design again.