Hi @shalini,
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:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
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:
https://powerusers.microsoft.com/t5/Flow-Ideas/idb-p/FlowIdeas
Best regards,
Kris