Originally Posted by
rwbuild
If painted or stained, use a utility knife to score the junction between the glazing bead and frame also at junction of glass but be very careful that the blade doesn't run off line. Then starting in the middle of the longest bead, gently prize the up adjacent to the fixing brads (should be able to see indents where the putty has shrunk at brad head) and work your way back to the mitres. Remove longest beads first the short ones.
You can also use a multi tool to cut the brads after lifting the bead about 1mm adjacent to each brad.