most people use a heat gun.
it is like a blow dryer.. but it blows hotter.
sometimes people put things in the oven to melt the glue.
other times they put the thing in boiling water.
and other times they hold the thing above boiling water to use the steam to melt the glue.
if it isnt only glue.. then there are probably plastic clips that need to be pressed with a screwdriver or something.
nobody of us would know where those plastic tabs are.
and if you have to break those tabs.. you could always glue the thing back together so it doesnt vibrate or come apart.
you just gotta be careful to not yank on the thing so hard that it comes apart 'much'
because you dont want to rip any wires from the speaker's terminal.
sometimes it is simply a metal plate with a spot of solder.
if there is any chance of the voice coil lead to be yanked from the speaker.. you dont want that to happen because the voice coil would be ruined.
a much easier task if you get the ear pads off.
you dont want to ruin those with heat or steam.. could start a disintegration process.
you dont want to get the speaker cone itself too hot or too wet from steam.. because dirt can stick to it or the thing could lose its flexibility.
i pulled some sony headphones apart and they had nothing but plastic clips.
luckily the thing wasnt glued together and none of the plastic tabs snapped off.
i think you either know exactly where the tabs are and use a special tool to take it apart.
you dive in and hope to get lucky.. while trying to be prepared to fix any mess you make in the process.
You know instead of trying it your self you can send them back to Sennheiser and they will fix them service them for a quite reasonable amount of cash that way you wont have to worry about ruining them.
Sennheiser are amazing by the way got some HD800's