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There is no officially supported method to downgrade an Ubuntu installation in this way. That would leave you with two options.
1.) Upgrade to a newer non LTS release (15.10 recommended) and in April when 16.04 LTS is released you can upgrade to an LTS version. I would recommend this as it is the least work, 15.10 is stable and you’ll be able to get to LTS in a couple months without much hassle.
2.) Create a new 14.04 droplet and copy over your files and configuration, then repoint any domains and delete the old droplet.
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