select anyone of the edges that were created between the face's old and new (extruded) position, and hit G(edge ring) to select them all
Go to Top Menu > Select > Store Selection to temporarily store that selection..
2 select the faces that were extruded out, and Move them back towards the mesh along the appropriate axis..
In this case the faces were extruded out with Extrude Region > Normal, so I'll move them back with Move > Normal
Zoom in to see clearly, and match the faces with their old pre-extrusion position as much as possible
3 if you've matched it up well enough, you can switch to Body/Object mode and do a Right Click > Cleanup, but if that doesn't work..
Go back to the top menu and re-select those (now tiny) edges.. Top Menu > Select > Recall Selection,
then simply Right Click > Collapse to get rid of them