You can use the resize2fs command to resize the file system to fill the available space after resizing the partition.

First, you need to resize the partition to use the available space. You can use the fdisk command to do this. Replace /dev/sda with another target if necessary.

sudo fdisk /dev/sda

This command brings up an interactive menu. Type p to print the partition table and make note of the partition number and start sector of the partition you want to resize. Type d to delete the partition, then type the partition number. Type n to create a new partition, then follow the prompts to create a new partition that starts at the same sector as the old partition, but ends at the end of the disk. Type w to write the changes to the partition table. If you are prompted to remove the signature, decline by typing n.

Next, you need to resize the file system to fill the new partition. Again, replace /dev/sda if necessary.

sudo resize2fs /dev/sda

Your partition should now be resized to fill the available space. Normally no reboot is required. Use df to make sure the changes have taken place.

df -ah