CAST (expr AS type) is a conversion function in OBIEE that changes the data type of a value or a null value to another data type.This is one solution to change the datatype,but when I tried with this the column heading in report changed to different colour and my client didn't liked that solution.So I tried another solution.Follow the below steps and screenshots.
1.Go to Column Properties tab
|Select column properties tab|
2.Select Data Format tab
3.Check “Override Default Data Format” box
4.Select “Date Format” field dropdown as custom/as required in your report. If you select “Date Format” field dropdown as custom then specify “Custom Date Format” field as required in report.