Complete uninstall of ubuntu 18.04 before reinstalling 20.04

I’ve run into some issues on Ubuntu 18.04 dual-boot when trying to install some gpu drivers and I wanted to get a clean reinstall of Ubuntu 20.04. I formated the partitions on which Ubuntu was installed via diskmgmt.msc on Windows 10 but ubuntu still appears in the boot menu and i get into GNU Grub bash-like terminal when trying to boot into it. How to completely wipe Ubuntu 18.04 before installing the newest version ?