Assuming you have WSL1:
If you don’t know how to use wget (for step 4), click on Download at the top of this page, then click on the latest ROOT release and download the (compressed) Binary version for your Ubuntu; then just unpack it to some folder and add this line
source /path/to/root/bin/thisroot.sh
at the end of your .bashrc file (of course, using your correct path to that file), as per the instructions. And do not forget that the X server (Xming or VcXsrv ) must be running every time you want to use graphics in ROOT or any graphical thing in that Ubuntu.
By the way, in step 3, the “export…” line should also be added to .bashrc, so you don’t have to type it every time.