The easiest way to install UTM is from the Mac App Store.
You can also download the latest stable version of UTM from GitHub.
UTM is and always will be completely free and open source. The Mac App Store version is identical to the GitHub version and there are no features left out. The only advantage of the Mac App Store version is that you can get automatic updates. Purchasing the App Store version directly funds the development of UTM and shows your support ♥.
If you want the latest bleeding edge build, check out the releases page for the latest beta build.
Open the DMG and drag the icon on the left to the folder on the right. To update UTM, you can just replace the existing installation and all your virtual machines will be retained.