You can't at this point in time with UTAU itself. I believe there is a plugin to do this however, from what I've seen it's very buggy and seems to be experimental.
If you're meaning "syncing" as a mixer to align the vocals with instrumental, I'm not the one to ask and I'm not sure which one is best. A lot of beginners use audacity. I you mean something else, please clarify.
Hello, and welcome to UTAforum!
UTAU by itself is only meant for the creation of vocals, which you will need to render (export as a WAV file) before you can apply further processing. I highly recommend using music software for the mixing process. You can find many free DAWs in this resource, as well as links to download free plugins to enhance the sound of your mix. https://www.utaforum.net/resources/songwriting-resources.181/