Transfer content to another user

To transfer documents, issues, assets, etc, from one user to another user, use the "Bulk Actions" function. It's available in all modules - look down the bottom of the list for a button labelled "BULK ACTIONS".

Start by searching for the items you want to transfer - in this case by Assignee (or in Documents, by Editor, or Approver).
For a user to appear in the search drop-down list they need to be an active user. If you're transferring content from an employee who has left the organisation you may need to (temporarily) make the user 'active' in order to transfer their items to another user. 
When you have the desired search results, click on the button at the bottom of the list -  "BULK ACTIONS" and the instruction will open up:

Select the items in the search results that you want to transfer (checkboxes will appear to the left of each item), or use the ' select all' link in the instruction. 

Note that 'select all' is only working on the current page of visible results. (At the bottom right of the search results you can change the number of results you see per page )

Then click on the second link under Bulk Actions, " click here to continue".

You'll see several options for actions you can perform on all the items selected, e.g. for documents:

There's another step for each of these actions, e.g., to choose which user should be the new editor or approver, or choose which tag to add/remove, etc. Make your choice and all of the selected items will be immediately updated. 
To cancel the bulk update, just reload or navigate away before you make this final choice, e.g. navigate away before you choose the new assignee. 
The other modules have similar bulk actions available.