Change username in Ubuntu on WSL

I am using Ubuntu terminal inside Windows Subsystem for Linux. I created a wrong username and I want to change it. I know I have to do:

sudo usermod -l newUsername oldUsername

but without being logged in. So I have created a new username using su - temporary to do it. But I can't kill the process with my old user name. I know it has a PID 6. I did kill 6 and sudo kill 6 but I can't get rid of it.