Get access to workitem fields (tfs api)

You cannot access the workmark fields described here: https://msdn.microsoft.com/en-us/library/dd997792.aspx

In particular, it is interested in how to reach and change the fields Remaining / CompletedWork. I do this:

TfsTeamProjectCollection teamProjectCollection = new TfsTeamProjectCollection(collectionUri); WorkItemStore workItemStore = teamProjectCollection.GetService<WorkItemStore>(); WorkItem workItem = workItemStore.GetWorkItem(id); workItem.Fields["Description"].Value = "test"; 

and here I would like to get access to CompletedWork and make workItem.Save() but with this there were difficulties since there is no such field.

Actually the question:

How to get and change the values ​​of these fields?