How to change dll reference in webservice

In my SOAP webservice, i am referencing a dll[.Net Project] ,which contains all the methods in the webservice. I changed data type of one of the method in my dll project ,rebuild it and refrenced the dll in webservice. But still,its pointing to the old dll reference and throwing errors like below,

System.MissingMethodException: Method not found: 'System.Collections.Generic.List`1<COS.IPEDS.Business.School> COS.IPEDS.Business.School.GetSchoolsByCipAndZipCodeByLevel(System.String, Int32, System.String, System.String, System.String ByRef)'.
at ShortTermTraining.findInstitutions.GetInstitutionsProgramsByCipCodeLevelZip(String zipCode, Int32 radius, String cip开发者_开发百科Code, String awardLevel, String ErrorMsg)

Is there any other place,should I check ?

Thanks, Vanitha