An MVP is usually the the first real version of your product, so it's only natural to find things to adjust or improve.
Thanks to our experience we estimate our quote with sufficient headroom to evolve and refine the idea on the go. We're happy to execute revisions to work within the initial scope but keep in mind that we need to limit revisions in order to guarantee that projects are delivered on time.
In case there is work to be done that exceeds the initial scope, we have to require additional payment. This will of course be reviewed in advance and confirmed by you.
Related article: What is the difference between a Bug and a Revision?