Statistically speaking, a simply local averaging filter would provide the maximum likelihood answer. However that will also blur true edges. There are other simple filters like median filter, Kuwahara filter (demo attached), as well as others for images that are considerably more sophisticated and better like BM3d, non-local means, K-SVD, K-LLD, UINTA, etc.
See this comprehensive overview by one of the leading denoising scientists: https://users.soe.ucsc.edu/~milanfar/publications/journal/ModernTour.pdf
