Assuming your install isn't encrypted (see oldfred's comment if it is), then you'll need to create a bootable ubuntu live USB with these instructions:
Then you will need to restart your computer, and force it to boot off the USB (it may do so by default, otherwise there will be a key combination or BIOS option to do this, but it will depend on your specific computer so I'd suggest googling how to change where it boots from for your specific computer).
Once the USB is booted, you'll first need to connect it to the internet (the same way you normally would). Then you should open a terminal and install gparted using
sudo apt install gparted.
You can then launch gparted by using
sudo gparted from the terminal, and resize your partition from there.
It is recommended you take a backup of at least any important data on the partition before going ahead with the resize as data corruption is possible, especially if you are moving a partition and not just resizing it.