You will need a sight pusher or a punch and hammer.
By telling you this I assume no responsibility or liability should you mess up and hurt yourself. Attempt at your own risk.
Top view of gun, Drift sight off the slide. In the dovetail will be two spring loaded plunger things. Remove the left one and put sight back on. This will remove the mag safety and the gun SHOULD function as normal except fire when mag is removed.
You could have a smith do it but only takes about 5 minutes to do it yourself.
Matt
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Matt's Custom Glock Resource - Because Stock Sucks!
http://home.talkcity.com/stadiumway/glockzilla/