I have a parent site , and two subsites deriving from it. However, I need to apply a master page template to one of the sub sites only, but I am unable to do so in SP 2013. I have tried applying the master page to a subsite using sp designer but that doesn't seem possible, I can however re-apply the default template e.g seattle, if the subsite has inherited a custom master page template from the parent site. What is the right way going about this ? Say in a more complex situation, with a parent site, having for example 25 subsites, how can I change that one subsite to use a custom master page, without resetting all the other subsites to the default template.
Browse to Site Settings page of the subsite and click on the "Master Page" under "Look and feel" section.
Select "Specify a master page to be used by this site and all sites that inherit from it:"
Select the master page from the dropdown shown in the following screen grab
Let me know if you have any questions