Should I install Visual Studio 2017 beside Visual Studio 2015 or should I first uninstall Visual Studio 2015 and then install Visual Studio 2017?

From page Visual Studio 2017 Platform Targeting and Compatibility

Compatibility with Previous Releases Installation

You can install and use Visual Studio 2017 alongside previous versions of Visual Studio, including Visual Studio 2015, Visual Studio 2013, and Visua Studio 2012.

So yes. you can install them without any problem.