Stop the swap

sudo dphys-swapfile swapoff

Modify the size of the swap

As root, edit the file /etc/dphys-swapfile and modify the variable CONF_SWAPSIZE:

CONF_SWAPSIZE=1024

To modify the swap file, edit the variable CONF_SWAPFILE, and run dphys-swapfile setup which will create and initialize the file.

Start the swap

sudo dphys-swapfile swapon