<p>I have this <a href="http://www.thingiverse.com/thing:1381474" rel="noreferrer">GoPro mount for a quadcopter as STL file</a>. It looks as follows.</p>

<p><a href="https://i.stack.imgur.com/qcZOR.png" rel="noreferrer"><img src="https://i.stack.imgur.com/qcZOR.png" alt="enter image description here"></a></p>

<p>How do I modify it so that it is wider and longer by a few millimeters but the screw holes stay the exact same size? Additionally the angle of the upper surface must stay the same. Please suggest the easiest solution for someone like me who as no idea about CAD software.</p>

<p>The perfect solution for me would be to just import it in Tinkercad and then modify it however I have no idea what the steps are.</p>

Many resources are available for modification using 3D CAD (including learning a tool such as sketchup, which is VERY beginner friendly). The easiest and fastest solution to your particular problem may be the
reddit community "3D Print My Thing"
which was created for EXACTLY this type of situation (help with modelling parts which will be 3D printed.) Another potential useful reddit community is
"3D Modelling"
which will have many people who are able to help you with this quadcopter/Go-Pro attachment 3D model.
Third solution: Thingiverse has a customize option for 3D models (including this one). have you explored using this interface to edit the model? This is most likely the easiest DIY solution.