To adjust metadata such as cache-control on an object in S3 without having to re-upload it and without having to use any third party tools, you can do the following with the AWS CLI. It copies the object to itself while overriding the metadata with your chosen settings:
aws s3api copy-object --copy-source <bucket-name>/<file> --bucket <bucket-name> --key <file> --metadata-directive REPLACE --cache-control "max-age=3600"
Process this command in a find to do it on an existing set of files that already exists in the bucket as you mention:
find . -type f -exec aws s3api copy-object --copy-source <bucket-name>/{} --bucket <bucket-name> --key {} --metadata-directive REPLACE --cache-control "max-age=3600"
replace <bucket-name> with the name of your bucket
WARNING: this will overwrite all your existing metadata on the files such as acl, just add additional flags to the command e.g. --acl public-read to set what you need.