system installation - How to totally remove Ubuntu and install Lubuntu?

I've Asus computer with 2x Intel Celeron CPU 1000M @ 1.80 Ghz.
I've installed Ubuntu 16 on my computer but it seems too heavy for it. When computer is opening sometimes it takes 5 minutes to prepare desktop and when I click on any application again it takes something like 4-5 minutes to run application.

Thusly I've decided to uninstall Ubuntu and install Lubuntu instead of it which is more lighweight. I got all personel folder to somewhere else and burned a Lubuntu ISO to USB stick.

Now I want to totally remove Ubuntu and all additional softwares which I've downloaded to computer and then make a clean install of Lubuntu. But I can not remove the Ubuntu.

I've checked internet and on every blog says something different. Finally I've run apt-get install lubuntu-desktop and now I can switch to Lubuntu but still computer to slow. As well whole pre-installed softwares are exist.

How can I achieve for a totally clean re-install of Ubuntu and installing a clean Lubuntu instead of it?

I have over came on issue which avoid me to format whole OS through BIOS settings . I had burned Lubuntu ISO to USB but couldn't see USB because of Secure Boot Mode was enable.

I've disabled it and then saw the Lubuntu installer USB on Boot Options and be succedd to totally removing Ubuntu, installing Lubuntu instead of it.

Thanks for @ciampix and @guiverc for their comments.