Firstly, turn the machine off, unplug from the wall, and let it cool down.
Get a small Philips Head screwdriver and unscrew the small screw which is holding the metal filter grid on in the brew head assembly. Don't force it as you may shear the screw off. Use a thin screwdriver to gently ease off the 1st grid. Ease the 2nd grid out with a thin screwdriver.
Ease the Group Head Seal out of the machine and dispose. Clean the 2nd grid of coffee residue, and also clean out the cavity of the coffee machine. Place the new Group Head Seal, ridge side up, on a flat surface, and place the 2nd grid into the seal, then replace the seal back into the machine. Take the 1st grid and clean thoroughly on both sides. Replace back into the unit cup side up, and re-secure with the screw, being careful not to overtighten.
NOTE: If you machine is more than a few years old, replacing the seal may not fix a leak at the group seal. The group handle may have worn down the group collar. If you feel up inside the group collar and find that the collar is "sharp" then it's likely that you may need a new Group Collar - a repair job which requires more than basic handyman skills.