See text messages in iCloud backup

You can't. See the following from Use Messages in iCloud - Apple Support

You can use Messages in iCloud on your iPhone, iPad, iPod touch, Apple Watch, and Mac. For your privacy, Messages in iCloud is end-to-end encrypted, which means you can't view or access Messages online via browser. 

Also note that you cannot view the contents of an iCloud Backup; you can only use the backup to do a full restore of a device or to setup a new device.