If you use SNAP the values are already shown as reflectances.
You just need to open the MTD_MSIL1C.xml in the product folder, not directly the jp2 files.
You can then export those values in several ways and formats.
If you wan’t to work with the jp2 files directly you need to devide the values in those by 10000. This value is the so called QUANTIFICATION_VALUE and is stored on the MTD_MSIL1C.xml.