You can do this a few different ways.
1. Use the "Recode" facility, which will allow you to either change the values in your existing column, or to create a new column with the binned values
2. Use the "Value Labels" column property, which will allow you to change the groupings for a column, without actually changing the stored values for the column
3. You can download the Interactive Binning Addin that Jeff Perkinson wrote
Interactive Binning
Jim