From your User Dashboard, click "Works (Datasets)" in the grey sidebar on the left, then find your work (dataset) in the list (you may need to use the pagination buttons to access the screen containing your work).
Click the black "Select" button to the right of your work. From the drop-down list select the menu item "Transfer ownership of work (dataset)".
The "Transfer ownership" screen is displayed, with a search bar for the user to whom you wish to transfer ownership. Enter some characters of the username of the user, and the username should appear in the search bar. Optionally add a comment in the "Comments" area below, and then click the blue button labelled "TRANSFER".
From your User Dashboard, navigate down the screen to the section called "Transfers of Ownership". In this section, you should see a sub-section called "Transfers Received", and here you should see the work that is being transferred to you. Click the blue button labelled "ACCEPT" and from the resulting drop-down menu, select the option "Allow depositor to retain edit access".