If Kenny is your 'grandfather', as an example, then you would go into Edit mode on your father, select the Add Father link on the right side of his profile, enter the Wikitree ID of 'Kenny', 'scroll to the bottom' so you can check to see if his spouse is listed at the bottom, with a checkmark. If that is the 'mother', then leave the checkmark and save. That will add both parents to your father. This can be done without Admin help because you are just connecting profiles to each other.
You need to send a message to Admin info @wikitree.com, telling them who you are, your relationship, that he has died, that you would like to take over management of his profiles, and then Admin will follow the process that Michel stated below.