GalaxyM01s volume down button stopped working

First thing to do is figure out whether it's the button you press with your finger, or the switch on a small circuit board that's causing the problem. On your phone the buttons are mounted on the back cover, so if you remove the cover you should be able to figure out what part's responsible. The lack of a click could mean the button isn't reaching the domed switch inside, or it could mean that little switch has failed.

If it's the outside button that's the problem, you can either replace it by itself or just buy a new back panel with the buttons already installed. Here's an example of a supplier that sells just the buttons (this is not an endorsment).

For Samsung Galaxy M01s SM-M01 10pcs Power Button + Volume Control Button(Black)

Be sure to verify the model number (SM-M01) matches yours to verify compatibility.

On the other hand, if you find that the metal dome under the button has broken or isn't clicking for whatever reason, then you'll need to replace the flex cable it's mounted on, that holds the two volume buttons. Here's one vendor selling that particular part, again check to be sure it's the right one for your phone.

Power On Off Button Volume Switch Key Control Flex Cable Parts For Samsung Galaxy M10 M20 M30 M40 M01s M02s M10s M21s M30s M31s - AliExpress

Note that the site sells both the power button flex and the volume button flex, so be sure to get the correct one.

Generally I include any available guides showing you how to make that repair, but I'm afraid there just isn't much out there for your particular phone. This video is specific to your phone and is meant for replacing the screen, but really all it's going to be good for is to show you how to remove the battery cover and possibly the battery if necessary.

samsung M01S FULL DISPLAY CHANGE -- SAMSUNG PHONE RESTORE - YouTube

The next two videos show how to replace the volume button flex on different Samsung phones, so hopefully one of them is similar enough to your phone that it will give you an idea how to go about it.

Galaxy S7 Volume Buttons Flex Replacement How To Change - YouTube

How to transfer Samsung side buttons flex cable and brackets #Shorts - YouTube

Hopefully that will give you enough information that you'll be able to fix your phone. Of course, you are welcome to come back and ask questions or seek clarification if you run into problems when you're making your repairs.

Good luck and be sure to come back and let us know what you find and how it all turns out; your experience could well help the next person who comes along with the same problem.