How to clean Smartcover?

I am told , but haven't tried it that a lint roller is a good way to clean the inside of the smart cover. As for the outside a wipe with one of those hygienic wipes seems to do the job nicely

Send it to the launderette for dry cleaning.

If it's really grimey, you could use a powerwasher. 😁

Seriously, if it's the leather cover, I'd use a leather cleaner/conditioner, same stuff you'd use on leather shoes or car seats.

If it's polyurethane, any household cleaner such as 409 or Fantastik should do the job.