If you want to log in to a different installed desktop environment, you need to indicate this.
- On the login screen, locate a "cog" icon.
- If you click the cog icon, you will see the installed desktop environments. Click the one you want to log into.
- From now on, your system will by default log you in in to the selected desktop environment after you provided user name and password, until you change your choice using the cog icon.
If you do not see your login screen, you must have automatic login enabled. Then, after your Ubuntu desktop appears, first log out. Click on the button in the top right corner, click your name and choose "Log Out". This will bring you to the login screen, where you can log in into the other desktop environment as I described. Next time, you will automatically land in your Lubuntu session.