How to unstick a row of shingles

How can i loosen a row of shingles so that I can slide a piece of flashing between the rows. I am trying to add a rain diverter to an existing undamaged roof. I tried inserting a putty knife (spatula) but the shingles are stuck together so tightly I am afraid I will damage them.

Much thanks for any help.

Either lay a bag of ice on them so the sticky gets brittle, or if that doesn’t work heat them up and you should be able to gently pull the seal apart. 160deg or so, like they get on a real hot day in the sun. Also you could try heating the blade of your putty knife up to melt the sticky. It can be a pain when they’re sealed well.