If it's a small crack, you can use a handheld torch on low heat, nothing fancy, clean the area with a wire brush, apply a bit of soldering flux, and use lead solder.... while applying low heat to the flashing, use the solder to fill gently in the crack. Do be careful. Too much heat will melt your flashing - if it's a larger crack, I would recommend some waterproof material such as roofing cement - otherwise, depending on the damage, a complete flashing replacement might be key - best of luck.

Lead is a very flexible metal, and you can close the crack by gently tapping the joint together with a small hammer. If you need to solder it, then you have to use "bar Solder" and keep the heat with a blowtorch low. Clen the joint until there is clean lead, and apply flux. Now, hold the bar solder on the lead crack, and heat the bar solder with a low flame, Until it starts to run. The heat on the solder will make the solder bond with the lead without heating the lead. Do not be mean with the solder and do a sort of welding joint with it. If you're not happy with the joint, you can always go over it again. This method is known as "lead burning". Do not use the standard reel of solder for this. It will not make a good joint as lead does expand and retract depending on the outside temperature. Using reel solder will open up the joint again as it expands. Finally, get a cloth and a large dab of flux to smooth the joint over. Good luck with whichever method you use. ( Bar solder is also known as "Tinmans Solder")

With a soldering iron, not a gas torch! You can use a gas torch with a soldering iron attachment, but you really only want the soldering iron part to touch the lead, or else you might melt holes in it and make your problem worse. Ideally, I would clean up the area, get any muck out of the crack, use a little flux, and, with my soldering iron, melt some lead-based solder into the crack. Then again, since that would be expensive even if I had a big enough soldering iron (and we are talking MAN SIZED to be any use), I am likely to buy a tube of gutter repair mastic from a DIY store and use that to close the crack. It is not as permanent as soldering, but it's a lot quicker, cheaper and easier, and it will be a close color match to the lead.

Rent an oxy-acetylene kit from your local hire shop and do it properly to get the best results
Even better still would be to replace the lead flashing completely.