In Chrome, Google search reverted to the old format. How do I change this back to the new format?

For Google Search Images, you can specify the parameter with different value in the given URL. Whether you need the new format or old format, it can be done as following.

New format

https://www.google.com/search?q=sine&sout=0&tbm=isch

Google Search Images - New result

Old format

https://www.google.com/search?q=sine&sout=1&tbm=isch

Google Search Images - Old result

Parameters explained

&tbm=isch is to switch search tab to "Images";

&sout=0 is to switch search result output, whereby 0 will show "New format" and 1 will show "Old format" as hinted in above screenshots.

Add search engine in web browser

Go to Google search page of either format. In the search text field, right-click and select "Add a Keyword for this Search..." (Firefox) or "Add as Search Engine..." (Vivaldi) or something similar.

The custom search engine may have this kind of settings:

URL: https://www.google.com/search?q=%s&sout=0&tbm=isch
Keyword or shortcut: gi (or any text of choice)

whereby %s is used instead of static text string

Additional information

The original information was found in this public thread (dated 2010) on Google Search and Assistant Help Forum. That has no mention of &tbm=isch parameter, which I had noticed by comparing URL for each type of search pages.

That being said, the parameter &sout=0 or &sout=1 makes no difference for other type of search pages. To this answered date, the parameter trick works only with Google Search Images.

Try the Chrome Cleanup Tool, described as:

This application will scan and remove software that may cause problems with Chrome, such as crashes, unusual startup pages or toolbars, unexpected ads you can't get rid of, or otherwise changing your browsing experience.

If that doesn't help, you could try to Completely Uninstall & Re-Install Google Chrome, by:

Uninstall Chrome via Control Panel / Programs and Features

Delete Chrome folders from the computer :

C:\Users\<YourUserName>\AppData\Local\Google
C:\Program Files\Google\Chrome

Install Chrome again.