It depends on the kind of backup you are looking for.
An S3 Bucket cannot be mounted on a server, which means that you won't be able to configure a "File" backup on it.
When it comes to backing up an S3 bucket, you should use "Storage Replication", which allows you to synchronize your bucket to another one (cross-provider).
This means that all your buckets will be mirrored, to another bucket on schedule.
Note that this is not considered a "backup" in the sense that we don't store any archive but a 1-to-1 copy of your bucket at a given time.