Follow below steps add SWAP space to an existing Linux Debian server:

• Create 2GB swap_file in /swap directory:
  dd if=/dev/zero of=/swap/swap_file bs=1M count=2048

• Setup file permissions:
  chmod 0600 /swap/swap_file

• Setup Linux swap area:
  mkswap /var/cache/swap/myswap

• Enable swapping:
  sudo swapon /var/cache/swap/myswap