Linux Shout published a tutorial about converting your existing Ubuntu Linux installation to Elementary OS 6.
How to try Elementary OS 6 early build on Ubuntu 20.04 LTS or Linux Mint
Convert your existing Ubuntu 20.04 LTS or Linux Mint to Elementary OS 6 ODIN by installing pantheon Desktop for Early build access
How to try Elementary OS 6 early build on Ubuntu 20.04 LTS or Linux Mint