A little known feature of Hit-Office is the ability to modify a group of elements.
When you are in a Hit-Office window (in offers, projects, service calls, …), you can select a set of data using SHIFT + click or SHIFT + the arrows.
Once selected, you may right click the selection with the mouse. Hit-Office will then offer you a list of possible actions. You can then select the field to edit and enter the value that this field should have. Hit-Office automatically apply your choice to all selected data.