code: public Observable<ManagedBackupShortTermRetentionPolicyInner> updateAsync(String resourceGroupName, String managedInstanceName, String databaseName, Integer retentionDays) {
          return updateWithServiceResponseAsync(resourceGroupName, managedInstanceName, databaseName, retentionDays).map(new Func1<ServiceResponse<ManagedBackupShortTermRetentionPolicyInner>, ManagedBackupShortTermRetentionPolicyInner>() {
              @Override
              public ManagedBackupShortTermRetentionPolicyInner call(ServiceResponse<ManagedBackupShortTermRetentionPolicyInner> response) {
                  return response.body();
              }
          });
      }
docs: Updates a managed database's short term retention policy.
  
  @param resourceGroupName The name of the resource group that contains the resource. You can obtain this value from the Azure Resource Manager API or the portal.
  @param managedInstanceName The name of the managed instance.
  @param databaseName The name of the database.
  @param retentionDays The backup retention period in days. This is how many days Point-in-Time Restore will be supported.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable for the request