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I do believe you cannot. The hosted service is a WYSIWYG type service. If you are looking to run a version specific instance of MySQL then you would need to spin a droplet up and install your desired DB/Version.
You can’t downgrade a MySQL version without having a nervous breakdown!
What I’ll recommend is for you to dump your data, remove the MySQL version you don’t need, install the new version you need and dump the data you have back in the new MySQL instance.
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