You may have noticed a prompt to take a sneak peek at the new Twitter layout in your desktop browser. It implies that you can easily change your mind after hitting the 'Take a look' button, so what's the worst that can happen?
Actually, having pressed that button, the first thing you'll notice is not the new design but the fact there is no easy way to revert to the old Twitter. There's no toggle, no option in the Settings, no nothing. It's probably an oversight, but right now that doesn't help you.
The good news is that the new design is merely applied in your browser, so opening your account in an Incognito window or on another computer will show you the old layout. And this means you can get rid of it simply by closing Twitter, deleting your cookies and cached data, then restarting your browser.
How to clear cookies and cached files in Windows 10
In Chrome click the More icon (three dots) at the top right and choose More Tools, Clear Browsing Data. Select a date range and select the box next to 'Cookies and other site data' and 'Cached images and files', then click Clear data.
In Firefox click the Menu button in the top right corner and choose Options, Privacy, Clear your recent history. Choose a date range, select the boxes next to Cookies and Cache, then click Clear Now.
Other browsers will have similar settings - learn more about removing cookies and cached data in Windows 10. Remember to restart the browser before reopening Twitter.