See below for how to update an existing IAM policy for granting permission to the S3 bucket. 

To update an IAM policy

1. Type "IAM" in the search box of the AWS console.
2. Select IAM in the search results to display the IAM dashboard.
   
3. Select Access management > Policies in the navigation sidebar.
4. In the content pane, click the + expand button for an existing IAM policy.
5. Click Edit.

6. In the policy editor field, add the following code to the Resource array.

   "arn:aws:s3:::$S3_BUCKET_NAME",

   "arn:aws:s3:::$S3_BUCKET_NAME/*"
7. In the code, replace $S3_BUCKET_NAME with the name of the S3 bucket selected when Creating an S3 bucket.
8. Click Next.
9. Click Save changes.