How do I update a Content Type Property (e.g Mandatory, Validation, Description) in Umbraco 6 (I'm using 6.1.6)?

I tried updating like below but it doesn't update it:

PropertyType prop = contentType.getPropertyType(propAlias);

//contentType is umbraco.cms.businesslogic.ContentType

prop.Mandatory = propAttr.Mandatory;

                prop.ValidationRegExp = propAttr.ValidationRegExp;

                prop.Description = propAttr.Description;

                prop.SortOrder = propertySortOrder;

                prop.Save();

Am I missing something?