https://www.researchgate.net/post/Converting_DN_of_Sentinel_2_image_to_Reflectance_values

https://www.youtube.com/watch?v=MM_I3NZibYE

Hi Royimani, in collaboration with our RECAREDD partners we (JRC) are developing a portable application called IMPACT.  It is doing what you are asking in 1 click.

See attached publication; much more info at http://forobs.jrc.ec.europa.eu/products/software

Enjoy and give us feedback

Technical Report IMPACT: Portable GIS toolbox for image processing and land c...

That is great news, Dario. There was quite a large learning curve to do this using the SNAP software in the Sentinel-2 Toolbox (link below). I look forward to trying IMPACT.

http://step.esa.int/main/download/ 

Being among the very first users of IMPACT as recommended by Dario, I can attest to the ease of use. It enabled my organization focus on information derivation from Landsat and Sentinel data and not image pre-processing. Conversion of DN values to TOA reflectance is a one step process.

Recommend it all the way. 

But I understand that the queation is Surface reflectance because Sentinel 2 a level 1c already has values un Toa reflectance

I recommend Firoz's answer

Best Regards