>>106323803
Just like non-anime. you get the best version available/possible. Then you use a good (interpolating) scaler if needed (e.g. lanczos), and maybe a filter if needed (for e.g. sharpening).
Doing anything beyond that, like using non-interpolating "upscaling" (i.e. introducing fake pixels with fake artificial math) is a sinful act, as it goes against the artistic intent of the authors.