Can I change master page only in subsite without selecting
"Specify a master page to be used by this site and all sites that inherit from it:" in "Site Settings" -> "Look and feel" -> "Master Page"?
Yes, you can through one of the below methods.
Using SharePoint PowerShell
$web = Get-SPWeb siteurl $web.MasterUrl = "masterpagerelativeurl" $web.Update() $web.Dispose()
Using SharePoint Designer,
- Open your Site using SharePoint Designer.
- Go to MaterPage section in the left side> Right Click on your Master Page > Select
Set Master Page as Default