Ubuntu 19.10更改桌面环境

Ubuntu 19.10使用GDM3作为登录管理器。此外，此登录管理器还允许您在登录期间登录到其他会话（“桌面环境”）。

• 确保在登录屏幕上选择了用户，以便显示密码。
• 输入密码之前，请单击“齿轮”图标以选择会话。

（Ubuntu 18.04的屏幕，但19.10中的屏幕与此类似）所选会话将在后续登录期间保持有效，直到您选择另一个会话。

For your particular case: your screenshot shows you are currently logged in. What you are showing is a lock screen. Your session is still active, but the lock screen protects your account so other people could not get access while you are away from the computer without having logged out. By default, it will kick in when the screen is blanked after a certain time of inactivity (configurable on Settings - Power). In Ubuntu, you can also lock the computer yourself with the hotkey Super+l). Thus

• Enter your password to return to your current session
• In the top right corner on the top bar, click the status menu button
• In the menu, click the entry showing your name. Then click "log out".

Now, you will have effectively logged out of your session. You will now have a cog to select another session for logging in.