Purchase or borrow an "Easyout" to suit
drill hole in broken bolt to suit easyout
insert easyout
unscrew broken bolt.

sometimes it is possible to remove the broken bolt with pliers/chisel/screwdriver after drilling a hole.
sometimes just the act of drilling the hole will drive the broken bolt out the other side.

if all else fails carefully drill to tapping size and clean thread out with a tap

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