This is currently not possible in the work item editor. the effort is tracked in enhancement 75165 (https://jazz.net/jazz/resource/itemName/com.ibm.team.workitem.WorkItem/75165)

You can, however, edit them in the my work view. First, enable "Schedule Risk Assessment" in the "Current Work" section's header menu.
After reloading the my work view, you can select "Show Work Time" from an items context menu. This expands the selected item inplace and reveals controls to set these estimates.


--
MikeS
Jazz Agile Planning team

pawrampe wrote:

On the Work Item, is it possible to see the Maximal and Minimal
Estimates on the work items themselves? The need is to set those
values outside of the plan because the plan does not allow for column
headers. I could not find a way to make this happen on a work item
presentation, nor could these values be edited from a WI query or the
'My Work View'.