There are these simple steps which you need to follow.
Step 1: Enable the Unknown Sources
This is a process in which you won't need to access the root or even a Nexus device, so you get to see the new Clock running on your Android smartphones and tablets too. but they have to be version 4.0 or later. Just make sure you enable the Unknown sources in your Settings, if it has not been done as default. Now download the new Clock app.
Step 2: Installing the Android M Clock App
Once you have installed apps from unknown sources, now you have to download the Android M Clock app which would be provided by the courtesy of APK Mirror.
Wait till it finishes downloading, then click on the notification tab to install it.
You have to Download the Android M Clock App that has this extension (.apk).
Step 3: Getting along with the Android M Clock App
This is just a mere information to share that all the updates in Google's Clock app has got the feature wherein you can choose the day which you would prefer the week to start with, like Monday, Saturday, or Sunday.
It has been done to encompass all the cultures of the world that start with different days of the week.
Hope that works for you.