The official SD Card Formatter recommended for installing NOOBS is unable to format an SD card with a previous NOOBS installation (despite its impressive and slightly scary version number of 126.96.36.199). You can, however, use Windows’ own
diskpart.exe utility to wipe the SD card:
DISKPART> list disk
Find the disk number N of the SD card.
DISKPART> select disk N DISKPART> detail disk
Verify the disk size to be absolute sure you’ve selected the right disk!
DISKPART> clean DISKPART> exit
Now the SD Card Formatter will accept the card. Maybe version 188.8.131.52 will be able to format SD cards even with existing partitions :-)