This guide details the steps I took to turn my Windows-Linux Mint dual-booted Thinkpad E14 into a Linux-only machine while preserving the existing Linux installation. It only applies if both operating systems are installed on the same drive and if the Windows installation is before the Linux installation on the disk.
In my case, my drive looked like this before any action was taken. The large partition on the left is Windows, the immediately adjacent one on the right is Linux.
This guide is based on steps found at this page: https://help.ubuntu.com/community/MovingLinuxPartition