Convert s3 video files resolution and store it back to s3
You can break the problem into 3 parts:

Get the file from s3: use boto3 & the AWS APIs to download the file
Convert the file locally: ffmpeg has got you covered here. It'll generate a new file
Upload the file back to s3: use boto3 again

This is fairly simple and robust for a script. 
Now if you want to optimize you can try using something like s3fs to mount your s3 bucket to local & do the conversion
I want to convert s3 bucket video file resolutions to other resolutions and store it back to s3.
I know we can use ffmpeg locally to change the resolution.
Can we use ffmpeg to convert s3 files and store it back using Django?
I am puzzled as from where to start and what should be the process.
Whether I should have the video file from s3 bucket in buffer and then convert it using ffmpeg and then upload it back to s3.
Or is there anyway to do it directly without having to keep it in buffer.