I just checked my JMRI installs on two Win10 systems both are running JMRI 4.17.6? and one has the uninstall? under 'Start', Apps ' JMRI ' the other does not show the uninstall for some reason and I suspect it has to do with WIN10 version.? The first, with the uninstall is on? Win10 Home N Ver1903. while the one NOT showing the uninstall is on Win10 Pro? Ver 1511 (this one refuses to update Win10, another mystery)..
You will have to go thru 'Programs and Features' or? 'Apps and Fearures ' depending on win10 version to remove JMRI.