This is a thick peach syrup, more of a peach sauce. But if you were trying to make a simple syrup and you ended up adding more water and the syrup is thin, then add more sugar to thicken the syrup. But if you do not want to thicken the syrup without adding more sugar, you can use thickeners like corn starch or corn syrup. Corn starch will whiten the syrup, so you can use something like a little bit of gelatin to thicken the syrup while retaining the flavor and color of the syrup.