If it's the stock launcher, you can't. Any app with a disabled uninstall button is usually a system app, and unless your phone is rooted, said apps cannot be uninstalled. Some allow rolling back to factory-installed versions (i.e. uninstall updates) but that's about it.
If it's a 3rd party launcher, while odd that the button is disabled, it could be that the app has been added administrator rights to prevent uninstalling. Try doing so by booting up into Safe Mode or checking the Administrators section in your phone's settings and disable anything related to said launcher. Again, if it's the stock one, it's a system app, NOT to be uninstalled (even if you root, getting rid of the stock launcher also disables certain UI elements 3rd party launchers don't have, so it would basically break your phone).