restore soft deleted item in database

Hello,

i am using the ci Model Class with $useSoftDeletes = true. Now i want to implement a recycle bin to my application.
Searching the deleted items works fine.. but i don't know how to restore these itemes?

I tried to update($primaryKey,['deleted_at'=>null]) or an update with complete user data in hope that an update deletes the delete timestamp...
But nothing worked. I searched in the documentation but there is nothing about restore.. just how to prevent deleting with soft_deletes

I hope someone can help me solving this problem...

best regards,
Niklas