Hi
I think you can use a simple bandmath like “B2/1000” and save as floating point data.
Please have a look at the algorithm document “https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-2-msi/level-1c/algorithm”. Reflectance, often between 0 and 1, is converted into integer values, in order to preserve the dynamic range of the data by applying a fixed coefficient (1000 by default).
Regards
Ralf