Increase size of encrypted swap

sudo swapoff -a  
sudo cryptsetup resize cryptswap1 --size (in sectors)  
sudo mkswap /dev/mapper/cryptswap1  
sudo swapon -a  

If --size is left out, it will use the whole device.

UPDATE - Based on feedback. Reboot and make sure mint--vg-swap_1 is still 980M.
IF IT IS, do the steps below. -- Try manually increasing the size of the container first (I don't think that should have been necessary). Your comment says you are trying to make the swap 4G (+3G), so:

sudo swapoff -a  
sudo lvresize -L+3G /dev/mapper/mint--vg-swap_1
sudo cryptsetup resize cryptswap1 --size 7812500
sudo mkswap /dev/mapper/cryptswap1  
sudo swapon -a  

On the other hand, if mint--vg-swap_1 is now 4G, then run this (allow for the size of the header):

sudo swapoff -a  
sudo cryptsetup resize cryptswap1 --size 7808404   
sudo mkswap /dev/mapper/cryptswap1  
sudo swapon -a  

Reference