Replacing a foglamp unit on a Vauxhall (Opel) Corsa Mk4 (E) is relatively straight forward. This procedure should work on all years between 2014 and 2019. This procedure will also allow you to replace the foglight surround trim if needed.
If you're flexible, then you may be able to reach through the wheel arches (by moving the liner out of the way), or even back around the undertray. For most, though, it's necessary to unhook the bumper.
To do this, you can follow the initial stages of my guide on Replacing the bumper of a Vauxhall Corsa E.
Removing the foglight
However you're accessing the foglight unit, it is held in place by 2 Torx screws (T15s)
Unscrew these and then pull the unit backwards out of the cowling
It may be that the cowling has been damaged or otherwise needs replacing, it's clipped into the bumper by a number of tabs, you can see some of these in the image below
To remove, carefully work these through, working around. It takes some patience, but comes out relatively easily.
Installing the Foglight
Reinstallation is the reverse of removal.
Put the foglight surround trim back into the bumper - it takes a bit of patience and care to line the tabs up, but you're essentially trying to slide the trim in away from the middle of the bumper.
If (like me) you bought a pattern part, you might start to think it's been made a few mm too small, but it will eventually go in.
Then, offer the foglight unit up to the trim, the screw holes will only align one way.
Fit the screws and tighten down, your foglight should now be attached to your bumper
Refit the bumper and remember to reconnect the foglight power cables when doing so.
Because you've disturbed them, you'll want to re-align your foglights, the adjuster can be reached through the hole in the front of the trim.