I have a dual boot (Windows 7 and ubuntu 21.04); I want to remove both OS’s and install Ubuntu 20.04 LTS version?

It’s really pretty straightforward.

The official guidance tells you how to do it:

https://ubuntu.com/tutorials/install-ubuntu-desktop#1-overview

You’ll need to download Ubuntu and create a bootable usb -

https://ubuntu.com/tutorials/create-a-usb-stick-on-windows#1-overview

I’ve always found that easiest to do using Rufus in Windows.

Then back everything up - you’ll lose any data on your drive in both Ubuntu 21.04 and Windows 7.

Power off the machine, then when you power it on with your newly created Ubuntu USB in (you may need to change the boot order to usb first), you can select to install Ubuntu and you’re given the option to install alongside the current OS’s or instead of the current OS (‘Erase disk and install Ubuntu’). You obviously want the erase disk option as you don’t want to keep any of your current OS’s.