You need to be the current owner of a company in order to transfer ownership. If this is not you then you will need to ask the owner to login and do this.
The person you are wanting to transfer ownership too needs to be added as a user to the company prior to ownership transfer. Find out how to add a user here.
To access click on your user profile, then click Account.
Scroll to find the company you are wanting to transfer the ownership of and Click Manage Users.
Here you will find a list of all users for this company, find the user account for the new owner and click, Transfer Ownership.
This will send an email invite to the new owner, once they accept, the ownership will transfer from your account to theirs. The invite is valid for seven days.
After accepting ownership the new owner will have seven days to setup the subscription for the company.
They can setup their subscription by following the instructions here.