I have already creted the invoice. Can I link the etimate once the invoice is already created and accepted?

Hi there, 

Welcome to the Community. QuickBooks is a versatile program that helps you streamline your work using the various accounting features offered in the program. Rest assured, I'll be happy to share more info so you're on the right track. 

QuickBooks is designed to only create an invoice from an estimate, hence, making the function you've described unavailable. I can see the benefit of having this function for your business and I encourage you to send feedback about this to our engineers. You can send feedback by clicking on the Gear icon and looking for Feedback. New product ideas and improvements are based on the feedback we get from users like you.

Let me know if you have other questions by leaving a comment below. I'll be a message away.