I am writing this because I couldn't find the way to set the default content type through the browser. If you know, please comment below.
The reason for doing this is to have your content type come up when the user selects new item. If you didn't do this, the user has to click the down arrow on the new item in the ribbon, and then select your content type. (Sometimes, this is just too much to ask a user to know how to do and why "Promoted Links" is so popular.)
It is actually quit ease with SharePoint Designer. You only need to navigate to your list, select the content type, then select 'Set as Default' from the ribbon.