This depends on how the original sight is mounted on the barrel.
Original type AR front sight assemblies are held on by two tapered pins, usually entering from the right side.
Many other of todays rifles have front sight-gas block assemblies held on by Allen screws.
The pinned sights are removed by getting the rifle well braced with wood blocks and using a punch to drive the tapered pins out.
It's best to get the pins started with a "starter punch" which is just a standard punch that's cut off to a 1/2" working length.
Once the pins are moving, switch to a standard punch to finish removal.
Then use a block of wood to gently bump the assembly forward and off the barrel.
Note the the tapered pins "usually" are driven in from the right side and out from the left side, but check the ends of the pins to insure they were not installed from the left side.
The Allen screw type is removed by just removing the Allen screws and bumping the assembly off.