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