You cannot change the buttons in an Approval Flow - Approve or Reject is what you get.
If you want to control the response buttons, take a look a the Send Email with Options action. This action allows you to define the buttons. On the other hand, it does not allow for a comment and does not use the Approval center.
Send an mail with option works same as start an approval.
can we use send mail with option instead of start an approval.
can it is possible to customize send an mail with option to have comment box.
Send email with options does not use the approval center and does not allow you to include comments - sorry.....
Do you want to add other buttons except Accept and Reject button within the Approval email sent via the "Start an approval" action?
If you want to customize other buttons within the approval email sent via the "Start an approval" action, I afraid that there is no way to achieve your needs in Microsoft Flow currently. If you would like this feature to be added in Microsoft Flow, please submit an idea to Flow Ideas Forum:
If you want to customize other buttons within your approval email, you could consider take a try with the "Send an email with options" action of Office 365 Outlook connector. But we could not add a comment within the approval email sent via the "Send email with options" action. If you would like this feature to be added in Microsoft Flow, please also consider submit an idea to Flow Ideas Forum: