The Set Syntax Atom package creates easy Command Palette entries for setting the syntax of the current file.
It is designed to make the transition to Atom easier for people migrating from Sublime Text by mimicking the method by which one changes the syntax of the current buffer in that editor.
Simply use the way you're used to in Sublime Text!
There are no key bindings set by default, though you could set keys for specific languages that you use often. For example:
Good catch. Let us know what about this package looks wrong to you, and we'll investigate right away.