If you are attempting to unmount a NFS command like

# mount -t nfs -o remount /mnt/nfs 
# umount /mnt/nfs 
# umount -f /mnt/nfs 
# umount -l /mnt/nfs 
# umount -lf /mnt/nfs

Identify which processes tied to the mount need to be killed by using lsof and fuser:

# lsof | grep /mnt/nfs

lsof command above identifies the PID of the processes associated with the /mnt/nfs share. Kill any processes locking the stale mount.

Try to force umount again after the processes as been killed

# umount -lf

References:

1. How to unmount a stale NFS mount that fails to unmount with ‘device is busy’ after network disconnectivity?