I'm currently working on a film shooting 5K red epic shots and a lot of 1080 HD footage's. Now we want to export the final product as a 2K film, what steps should I take in color correcting, should I color correct and edit the footage as 5K and then render as 2K, or should I take the 5K render it to 2K and the color correct and edit the footage ?
Generally the film, from the beginning, is converted to an intermediate/mezzanine codec like ProRes or Avid DNxHD and is traditionally HD (1080x1920 as this is Avid's highest resolution).
These intermediate files are then edited. Once editing is locked, the film is split into "reels" of about 15 minutes. Why? If you have to re-edit a portion then you only need to change a piece of what is going to the next step of post, rather than possibly disturbing edits within the timeline of the entire film.
Once in reels, footage is organized and sent to audio, vfx, and so on. Once these steps are finished, the film is ready for "finishing".
Finishing will happen in applications like Scratch, Baselight, Speedgrade, or Resolve amongst others. They're commonly considered color grading apps although the toolsets are more extensive.
The edit is then matched with the original 5K edits or, in the case of visual effects 2K renders, replacing the original intermediate HD video from the first step. Footage is then color graded and exported to the finished 2K result.
That's in a nutshell, as some films will have multiple post steps going on at the same time.