if you need to keep your old one than best is to get a ring spanner that has a slot in it (good quality one, china ones will just bend and round the bolt) or i have seen a special tool for the job as well, sort of like a socket with a slit in it like this
http://www.autobarn.net/lis12100.html
using an open ended spanner if its seized can make the problem worse but rounding it
if you don't need it anymore, cut the wires and use a socket
and as mentioned before, needs to soak in wd40 or the like and use a breaker bar if you don't have enough leverage