Right-click on the ext4 partition you want to resize and select “Resize/Move.” In the Resize/Move dialog, you can adjust the size of the filesystem by dragging ...
You cannot resize your / partition booting normally - to make changes you have to unmount it and you cannot unmount your / partition. Boot your mint install ...
Just boot your system as normal, then from a terminal window do: sudo lvextend -L+20G -r /dev/mapper/centos-root # resize the LV and the underlying fs.
Some filesystems can be resized when mounted and ext4 is one of them, but you don't want to just resize it, you also need to move the entire partition to the ...