Some front sight pins tap out pretty easy, some are a total bitch.
The gas block needs to be supported in order to remove the pins. Do NOT just put the upper in a vise using a receiver block. The end of the barrel will have too much flex.
We just took a piece of white Lexan and milled a slot for the barrel to lay in and then cut a relief for the front sight. Drill a couple of holes for the roll pins to drop into when they come out.
Remove your upper from the lower. Remove the handguards. Lay it on a good study bench in the fixture. Align the pins with the holes in your fixture. We use a nail set punch to get the pins started out and finish with the proper sized punch. The pins are tapered and generally remove from left to right. Use a punch the size of the small end of the taper pins. If you use too large a punch it will get stuck good and tight and you will be cussing.
The reason for the nail set punch is that the pins are rounded on the ends and the carpenters punch has a cup in it and is less likely to slip off the pin end and mar you front sight base.
If the gun has been assembled for years or shot a lot you may want to soak the pin area overnite in a Liquid Wrench type solution.
You can make the fixture from any good quality hard wood such as oak if lexan is not available.