My aim is to carry out fusion between Sentinel 2 and UAV images. Sentinel images are already converted to reflectance values but UAV images are in DN number format. How can I convert UAV DN values to reflectance for this purpose?
Converting UAV DN values to reflectance can be done in two different ways, of which only one is likely available to you.
The first approach is to include the atmospheric correction in the survey design, meaning that you bring supporting equipment like reference panels or irradiance measuring sensors to account for the incoming light at acquisition. See this paper for more on this. The aforementioned paper develops a atmospheric correction module for drones and also attempts to make it work without proper supporting information from reference panels or irradiance measurements, but there is still some uncertainty left.
Another approach, which is easier, as it doesn't require fundamental changes to the field work, but also more uncertain, is to do a pseudo correction of your image frames by doing histogram matching between your high resolution drone DN values and the atmospherically corrected S2 data. This may cause quiet a few challenges, due to the very large difference in spatial resolution, so it may be safest to only apply a rough normalization, focusing on the 25th, 50th, and 75th percentiles.