I have a sharepoint 2013 portal. When I go to the site settings I normaly see the option "Masterpage". I dont see it at this moment. What can I do to change the masterpage?
You will only see the option "Master Page" if the site is a Publishing site (having publishing features enabled)
The Url to the master page settings is
/_layouts/15/ChangeSiteMasterPage.aspx which will throw an error on for example a Team Site
enable publishing feature from site settings. Goto site settings >> Site Collection feature and activate SharePoint Server Enterprise Site Collection features this feature.