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:

1. Disable the swap file and delete it (not really needed as you will overwrite it)

   sudo swapoff /swapfile
   sudo rm  /swapfile
   

2. 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
   

3. Assign it read/write permissions for root only (not strictly needed, but it tightens security)

   sudo chmod 600 /swapfile
   

4. Format the file as swap:

   sudo mkswap /swapfile
   

5. 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).