Could someone please explain step by step how edit the selection-pattern of a Group SOP? (Not by modifying the number string, but work through the viewport).

The simplest way is to simply make your selection then click the group icon on the shelf. If you don't want to edit the numbers in the group pattern field then simply throw the entire node away, remake your selection and click the button again.

Have you watched the video on selection?
https://vimeo.com/113102820 [vimeo.com]

Enivob, thanks, but I want to know how to edit the pattern without discarding the group SOP.

In terms of workflow both discarding and manually editing the numbers seems backwards, especially given that most SOP nodes have an arrow-icon next to the pattern field that lets you edit the selection mask with all the great selection functionality of Houdini.

Discarding the selection means re-selecting everything again. For some reason, the arrow-icon does not exist on the group SOP. I though there might be a special workflow, pressing ENTER while on a Group SOP will highlight its selection-pattern in the view-port, however additional selection or modification seems to disregard the highlighted pattern and pressing ENTER again (to confirm) will just create a new group SOP in the node graph with a new group name..

.. any advice.. ?

Select the group sop, go to the viewport and hit the ` key to enter reselection mode.
I don't have an english keyboard, so for me it's the ^° key on the top left above TAB.
You can also reassign it in the hotkey manager. Search for “Restart Selecting”.

But I wholeheartedly agree that this node needs a visible action key next to the pattern field.