To upload any contents to an S3 bucket, you need to enter the credentials of the bucket in the S3 browser.
If you are uploading the files to a bucket in your AWS account, ask your AWS system administrator for the bucket credentials.
If you are uploading to the S3 bucket in the Mithi account, then you need to refer to the mail received on your registered email id when you signed up for LegacyFlo.
The mail sender is "LegacyFlo Notification" and the subject of the mail is "LegacyFlo credentials". The mail will have an attachment with the credentials of the S3 bucket.