AT:

• in process instance details view
• only the variable value can be edited
• variable value editable directly in the table
• confirmation dialog before updating value in db (confirmation triggered when edit field is left)

HINT: I like the in-line editting way of JIRA - maybe we can do it in a similar way, also see screenshot
(maybe this link helps: http://vitalets.github.io/bootstrap-editable/)