Need to change key cylinder

Hi,
Is this on a door or ignition - or tailgate?

It will need the removal of plastic trim in any case. For the tailgate and door locks, remove interior trim, access the mechanism from inside, removing connecting rods first - then sliding back the retaining clip ro remove the lock. Sometimes you release the clip first - then unhook the connecting rods. Replacement locks should be fitted in the reverse order.

If it is the ignition cylinder - Remove plastic shroud, insert key, then rotate key to position one - steering lock off. Insert a small tool into the side of the cylinder to push in the retainer - then the lock cylinder can be removed still attatched to the key. A new cylinder is inserted in the same way.
If you do not have a key that fits - call an experienced locksmith - as this is a specialised job. DIY attempts can actually wreck the entire steering column - rendering it undrivable - needing complete replacement.