1.) Navigate to the "Permissions" tab on the main page of your bucket. 
2.) Click on "Bucket Policy"
3.) Copy paste this code snippet in. 
{ 
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "PublicReadGetObject",
            "Effect": "Allow",
            "Principal": "*",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::YOUR-BUCKET-NAME/*"
        }
    ]
}
4.) Replace "YOUR-BUCKET-NAME" with your bucket name
5.) Click save. Now all objects will be default by public