http://sqlinthewild.co.za/index.php/2008/11/04/what-are-statistics-and-why-do-we-need-them/
You can't change the frequency. The % of changes that kicks off a stats update is hardcoded. You can update stats manually using UPDATE STATISTICS.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability