increase root partition size

I created an ec2 instance and set the root device to 120GB but only 20GB shows up in the parition:

[root@xxx.xxx.xxx.xxx ec2-user]# lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
xvda    202:0    0    1G  0 disk
ââxvda1 202:1    0  949M  0 part
xvdb    202:16   0  120G  0 disk
ââxvdb1 202:17   0   20G  0 part /

How do I add the remaining 100GB to the / partition?