Yes, you can re-order Custom Fields. The new order will be immediately visible in Item Details view. If you want to change order of Custom Fields in Item detail view, change their order in Custom Field listing.

Go to Main Navigation Setting-Menu-Icon Administration > Configuration > Manage Custom Fields.

Reorder-Custom-Fields
  • Locate Custom Field in the list.
  • Reorder the list by dragging the Custom Field drag and drop Move-Icon icon.