From Ubuntu 18.04 onwards, a swapfile rather than a dedicated swap partition is used. The swap file is named "swapfile". To change the size of this swap file:
Disable the swap file and delete it (not really needed as you will overwrite it)
sudo swapoff /swapfile
sudo rm /swapfile
Create a new swap file of the desired size.
Determine the size of your swap file. If you want to make a 4 GB swap file, you will need to write 4 * 1024 blocks of 10242 bytes (= 1 MiB). That will make your count equal to 4 * 1024 = 4096. Create the file of this size with the command
sudo dd if=/dev/zero of=/swapfile bs=1M count=4096
Assign it read/write permissions for root only (not strictly needed, but it tightens security)
sudo chmod 600 /swapfile
Format the file as swap:
sudo mkswap /swapfile
The file will be activated on the next reboot. If you want to activate it for the current session:
sudo swapon /swapfile
You can check the swap that is available with the command
swapon -s (no root permissions needed).