You can make changes to your logo anytime by going to your logo download page and clicking the "edit" link. On desktop, the edit link is found in the left sidebar of the page:
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On mobile devices, your edit link will be at the top of your screen.

After clicking "edit" you'll be taken back to the logo editor and can make any changes you like by selecting the blue tabs found on the left side of the screen for desktop devices...
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...and on the bottom of your screen for mobile devices.

When finished editing, click the "Free Download" button located at the top of your screen to save your changes and return to your logo download page.