There are people who specialise in doing them, reasonably easy to find. DIY is possible but if that's not your bag it should be quick and cheap to pop it out and replace the glass.
Yes restored them ourselves, stripped, replaced rotten wood, re-weighted, draft proofed, sand masticed (previous owner had an affinity for silicon sealant and everything was painted shut)
While restoring sash windows we've broken a pane and it was pretty easy/cheap to replace it.