Mutating File at S3 Bucket w/ AWS Lambda

Given an S3 file at s3://path/to/bucket/file, I'd like to execute an AWS Lambda Function that appends "BAR".

To clarify, let's say that a file containing "FOO" lives at s3://path/to/bucket/file. I'd like to execute a JavaScript function that appends "BAR".

So, after mutating the file at the S3 file location, the contents of s3://path/to/bucket/file would read, "FOOBAR".

How can I do this?