There are two ways to do this.
First, on your Glean new tab page, you can click on the Settings icon in the top right corner toggle off the "Use Glean as new tab page" setting.
Alternatively, you can go to your Your account page (hover over your avatar in the bottom left corner and click "Your account" in the menu). On the Preferences page, Under "General", toggle off the setting "New tab page."
And if you have a minute, we'd love to hear why you preferred your existing tab page so we can understand where to improve! Get in touch via the Messenger 👍