How to send sms to all contacts in Android 6? I need to inform all the contacts with mobile numbers in my contacts list on my new mobile number. How do I do that?
Most stock messaging apps are not able to do this... if it does, create a new message and go to groups and select All Contacts.

If it does not have that capability, there are several alternative apps in the Play Store that do have that capability, such as Textra SMS, Chomp SMS, Handscent Next SMS, or most other 3rd party SMS/MMS apps are capable of this as well. 

Note that in Android 6 there are some permissions involved and only one messaging app can be active at a time, so you would have to install the new one, give it permissions to be the default messaging app, send your group message, then change the default messaging app back to the native one that came with your device if you wish. Although, depending on your stock messaging app, you may find a 3rd party messaging app much more feature rich and you can safely leave it as the default.