Deleting matches is absolute. That is; It is not possible to undo deleting.
You do not need to delete matches before scheduling. If you already have scheduled group B12-A, and you want to reschedule, it is no point in deleting the existing matches first.
You can delete a whole schedule by using the menu as shown (bubble 3):
You can multi-select matches by using the CTRL-key while you left mouse click on the boxes (matches). TIP: You can select a block of matches by pressing SHIFT-key while dragging a rectangle around/over the matches.
After selecting matches, just right click and choose "Delete"
It is also possible to delete matches from this table. Remember to select matches in the upper left checkbox. TIP: Make a filter first if you i.e. want to delete all matches in a category. See arrow.