It will depend on in what form your tree is backed up. If it is as a file on your device do the following:
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Go to the account you want to export the tree to.
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Go to 'My Account', and click on 'Create or import new family'.
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Click on 'Import GEDCOM or FamilyScript'.
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Click 'Browse' and find the tree's file in your documents where it is saved.
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Click 'Import', and then 'Finish Import'.
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Save the tree on the new account (use the normal 'Save' button).
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You should then be able to use the tree as normal.
If the tree is on a USB stick or similar, download it first and then follow the above steps.
Hope this helps!