I have the same problem with editing part quantities - but it appears to be a bug linked to the page number! I have multiple part lists, and my page is set to view 496 parts.
If I follow BUWHITE's instructions, it works fine - but only if the part I want to edit is on "page 1" of my parts list. If I click on the list I want to edit, then select "page 2" or "next page" to find my part, clicking the part does not bring up the Part Summary and hence I cannot edit the quantity of the part. Clicking on the list again gets me back to "page 1" (and part quantities are editable), click on "page 2" or "next" means I can't edit the part quantity. VERY frustrating!
Has anyone else seen this?