Create a GParted live bootable USB drive
I've tied to resize my main partion, I had to unmount it to do it.
https://gparted.org/liveusb.php
You just have to follow the steps.
Boot on the USB drive
You have to change the boot order.
I can access to the bios settings without typing on every keys who start with F*.
I have an option to do it at startup: UEFI Firmware settings.
Now go to Boot and set the USB drive at the top to boot on it first.
Gparted
Select the first choise to boot on it.
Launch GParted and you are ready to resize the partitions.
Be carefull you can loose your datas