Downgrade an app in Nougat
This installs the old version without removing the new one: 
adb install -r -d /path/to/old.apk

The -d flag is to allow a downgrade. -t allows test packages. 

You can also uninstall the application, then install the old apk. This may be necessary, for example, if the application data has changed format in the new version and is not backwards compatible.