I have added 7 camera's, called cam1 to cam7 and the camera's appear in reverse order on the BlueIris screen. It seems not possible to arrange the camera's in different order.
The camera's are added to 3 groups ea group 1 which contains all odd-numbered cams, group 2 contains all even numbered cams and group 3 which contains all cams. The group 1 cams appear correctly ordered as cam1 - cam 3 - cam 5 and cam7, the group 2 cams are reverese ordered as cam6 - cam4 - cam2
Any idea howto reorder the cams so they appear at the diserde order on the screen?

Select the group, next to that drop down there is an edit button labeled edit layout.

That "Edit layout" button has many features that you need to be familliar with.
Like @louyo stated, click it once and your screen border turns green and you can drag the cameras to whatever position that you want.
Also when the border is green, you can right click and get a menu for many more features such as to toggle hidden cameras, disabled cameras etc.... Once you toggle a menu and/or move your cameras around, you must click "Edit layout" again to return to normal functions.