Install fdisk and resize2fs
use fdisk to show the partitions
write down the starting sector address of /dev/sdb2 (which is the root partition)
use fdsk to delete the partition 2 (which is sdb2), don't write the changes to disk yet
use fdisk to create a new partition 2, choose/type the starting sector address you wrote down earlier (as by default it will try to place it somewhere else), and leave the default end sector address (this will mean the partition will now use all available space)
write the partition table changes to disk. It may complain about partition signatures already present, write n to NOT remove the partition signature to proceed.
write resize2fs /dev/sdb2 to enlarge the actual filesystem to fill the larger partition size.