Remove the two bolts (small arrows) then lift the heater slot cover (black plastic) from the side where the big arrows is pointing. You can then remove the heater cores (2nd picture). They can be turned out without bending any of the tabs. The same is true of the temperature sensors. The caked on (and toasted) dust can simply be vacuumed off.

You are done. Reverse the step to reassemble. I recommend white electric tape to put the part back on that you had to remove with the knife.