There was a thread about this a while ago
https://www.woodworkforums.com/showthread.php?t=47820

I've been doing a few more of these recently and I've worked out a few more wrinkles.

If you just are wanting the putty to look smooth for repainting then this is what I do now. I sand the putty with a 40 grit paper on a Bosch delta shaped sander (Bosch PDA 240E). I hold a broad (100mm) putty knife next to the glass to stop it getting scratched by the sander. Sanding sheets don't last long. Then any cracks in the putty I dig out with a Dremel tool with a tungsten carbide bit. Then fill with Nordsjo Super Filler, including a skim coat. This filler is slightly flexible and is sandable. It can also be worked smooth with a wet putty knife.

If you want to remove the putty completely then the best way is to cut between the steel and the putty with a Stanley knife with a new blade. Blades will go blunt very quickly so you need a packet of 10 or more on hand. Once the blade goes blunt (it might last 5 minutes if you are lucky), this becomes *much* harder so if you haven't tried this with a brand new blade, give it a go.

Then you can try and stick a steel rule or putty knife between the frame and the glass (from inside the window) to help get the glass out.

Any really stubborn bits you can have a go with a Dremel tool with a 1.5" EZ-Lock cutoff wheel (I used a metal cutting wheel - I didn't notice it was a metal cutting wheel when I bought it but it seems to work pretty well and lasts very well). It's also good for cutting off any metal pins embedded in the putty.

I also tried using the Dremel tool and the 1.5" cutoff wheel to cut between the glass and the putty (you can't get it to cut next to the steel because of the angles). It worked well at this task, but it is not much use in the end because the putty was only removed from the glass and not from the steel. You have to break the glass to get it out, then you can remove the rest of the putty more easily. I also tried a heat gun which softens it a bit.

The method with frequently changed Stanley knife blades I used on the next two window panes was much quicker and cleaner. I had the job done in about half the time and these panes longer and skinnier so had much more putty to remove.

I also fixed a wooden window recently. It had a rebate 6mm x 10mm or so. The steel windows have a rebate of 15mm x 20mm. No wonder the putty is hard to get out. There's gallons of the stuff.