Unfortunately, the account used with the Flow approval action will always shows as the "created by" in the approval email. You can add someone else as the "Requested For" name = to do so, click on Advanced Properties in the Approval action and provide an email address for the Requestor.
Changing this address is a highly requested feature, but for now it does not exist. At this point, whichever account creates the flow, also sends the approval emails. However, you can do custom approval flows (not using the built-in approvals functionality) that allow you to use other accounts. I discussed those options here yesterday - how to remove flow owner in approval email.
Also, as @ScottShearer pointed out in that same thread: "A simple way of sending an email and not having the Flow authors email be used as the "from" address is to use the "Send an Email Notification" action - it will be sent from "PowerApps or Flow". The action has a green envelope as it's icon. No additional configuration is required."
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