For bare metal restore of a cell, check the owner's guide for Exadata. In the latest versions, it's in chapter 7. To reimage a cell, you'll utilize the internal USB recovery media. The USB recovery media contains backup copies of the configuration for that specific cell, so that when it's finished, you don't have to go back and reconfigure the cell when it's done. Note that this should only be performed at the behest of Oracle support, as I've seen a couple of bugs in the restore procedure that made it pretty much unusable on older versions (below 220.127.116.11.x).
More importantly, what's the problem you're facing that is making you have to reimage a cell?