How to delete a partition using fdisk?

I am trying to install a GSI that is 1.8GB, and I need to re-partition the system partition to allow the img to be installed. I tried using fdisk /dev/block/mmcblk0, then I tried using the command d, but all it says is "Selected partition 1" and it doesn't allow me to choose the partition number, which is partition 26 in my case (userdata). How to delete a partition using fdisk?