How to choose (Mapping) a Master page in SharePoint Site

Steps

•  Site Settings for the designated site, under the Look and Feel section, choose Master Page.

•  Site Master Page Settings, select one of the two options for the Site Master Page or System Master Page sections.

• Inherit site master page from parent site Choose this option if you are configuring a child SharePoint site and want to use the parent master page.
  
  
• Specify a master page to be used by this site and all sites that inherit from it Choose this option if you want to map a specific master page to the site, or if you want to map a specific master page for administrative pages. A drop-down box named Default or All Channels is available for you, depending on your site or system configuration, so you can select a specific master page stored in the master page gallery. Select the desired master page from the drop-down box.

Refrence:msdn