As root.

fdisk /dev/mmcblk0

Delete the second partition /dev/mmcblk0p2

d
2

Create a new primary partition and use default sizes prompted. This will then create a partiton that fills the disk

n
p
2
enter
enter

Save and exit fdisk.

w

Now reboot. Once rebooted:

resize2fs /dev/mmcblk0p2