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! Bring up the Command Palette and use the fuzzy search to enter the syntax you want. See the Atom Flight Manual for instructions on how to show the Command Palette on your platform.
There are no key bindings set by default, though you could set keys for specific languages that you use often. For example:
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