Transfer content to another user

To remove a user, go into Settings and make them inactive

To transfer their assigned documents, issues, assets, etc, use the " Bulk Actions" function. It's available in all modules. 
In Documents, find the documents you want to transfer e.g., search by Editor = Fred.
Then look down the bottom of the list for a button "BULK ACTIONS" and click on it to see:

Use the ' select all' link or just choose some items in the search results (checkboxes appear to the left of each item).

Then click on the link under Bulk Actions, " click here to continue".
You'll see several options for actions you can perform on all the documents selected:

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.
The other modules have similar bulk actions available.