Steps :
1- Go to window start bar and search Microsoft store.
2- Then search ubuntu version as per your requirement.
3- Then after select require ubuntu version click on get, after that it will take few minute to download and then install.
4- It will take few minutes to install.
Other Important task.
1- You can see all version of ubuntu install in wsl using below command.
open powershell, and run this command wsl --list --verbose(it will list all ubuntu version you have in wsl)
2-How to switch between version.
wsl -s Ubuntu-20.04(For select Ubuntu-20.04)