Someone has created Master page using design manager. Now I want to modify this master page to make some changes.
Which is the best way to do it without breaking the original master page?
Copy your existing masterpage html file to same directory with another name. New masterpage automatically will be created. Change site master page to new one and modify its html, as you want. Masterpages stored in /_catalogs/masterpage folder of your site.
If you would like to continue the development further on the master page, I would suggest export the design package and import and apply in a developer site.
Production -> Export Package -> Developer Site -> Import Package -> Make Changes
Customize the new developer site and export and import the package from the developer site to the production package.
Developer Site -> Export Package -> Production Site -> Import Package
This will give you a site to test and continue working for any future changes.
Thanks and hope this helps Deepak