From Portal:
Configure your RDS backup retention policy to be at least 7 days.
1. Sign in to the AWS Management Console and open the Amazon RDS console at https://console.aws.amazon.com/rds/.
2. In the navigation pane, choose Databases, and then choose the DB instance that you want to modify.
3. Choose Modify.
4. In ‘Backup’ section,
a. From the ‘Backup Retention Period’ options set the retention period to at least 7 days.
b. Select ‘Start Time’ and ‘Duration’ in ‘Backup window’, which is the daily time range (in UTC) during which automated backups will be performed
5. Click ‘Continue’
6. On the confirmation page, select ‘Modify DB Instance’ to save your changes

From TF:
resource aws_db_instance” “default” {
allocated_storage = 10
engine = “mysql”
engine_version = “5.7”
instance_class = “db.t3.micro”
name = “mydb”
username = “foo”
password = “foobarbaz”
parameter_group_name = “default.mysql5.7”
skip_final_snapshot = true
+ backup_retention_period = 7 # number should be 7 or more
}

From Command Line:
aws rds modify-db-instance –region REGION –db-instance-identifier DBINSTANCE –backup-retention-period 7 –apply-immediately

References:
1. https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithAutomatedBackups.html
2. https://aws.amazon.com/rds/faqs/
3. https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/db_instance