How to change mount volume on AWS

I have two volumes in my AWS ubuntu instance:

NAME  MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
xvdb  202:16   0    40G  0 disk /mnt
xvda1 202:1    0   400G  0 disk /

I want the 400GB one to be the main one that the server uses. However, this is currently not the case:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       32G  3.5G   27G  12% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            3.7G  8.0K  3.7G   1% /dev
tmpfs           752M  208K  752M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            3.7G     0  3.7G   0% /run/shm
none            100M     0  100M   0% /run/user
/dev/xvdb        40G   49M   38G   1% /mnt

How would I mount the big volume mount as the active one?