Following these steps will erase all existing content on the microSD card.
First, insert the microSD card into your computer.
You need to install the right Raspberry Pi Imager for your operating system. You can do this on ubuntu by running:
sudo snap install rpi-imager
Or on other operating systems follow these links:
Once this is done, start the Imager and open the “CHOOSE OS” menu.
Scroll down the menu click “Other general-purpose OS”.
You will then be able to see a list of Ubuntu downloads to choose from. Select the “Ubuntu 20.10 Desktop (Raspberry Pi 4)” option. As indicated in the imager this only works for the Raspberry Pi 4 with 4GB or 8GB RAM.
Select the image and open the “SD Card” menu. Select the microSD card you have inserted, and click “WRITE”. Then just sit back and wait for the magic to happen… (This magic might take a few minutes).
Now you have your Ubuntu SD card. Before going on, make sure your Pi is off and insert this SD card. This is what the Pi uses to load all the software you’re about to use.