Big problem for me, hope the fix is easy. A year ago in 16.04, I followed some bad advice and could not boot. I ended up using the terminal to install Lubuntu, and after that the Lubuntu icon would appear during boot, but Ubuntu always came up as before. I was hoping when I upgraded that this problem would disappear, but alas it has not. I cannot start up unless I select lubuntu at the password sign in. I am in Lubuntu right now. After upgrading to 18.04.3 I was using ubuntu, but now it is the next day and when I restarted, my password will only boot if I select lubuntu. (which I have never used). So wondering if I can use terminal to install ubuntu boot command and delete lubuntu boot command. Thanks in advance if someone can help.