You can use the Field Calculator to convert field values stored as Degrees-Minutes-Decimal Seconds (DMS) to Decimal Degrees (DD).
For example, 953015.75, broken down to its constituents, is 95 degrees, 30 minutes, and 15.75 seconds. For the sake of this example, let's assume we have two fields in the table: one named LatDMS, which stores the Latitude values, and another named LongDMS, which stores the Longitude values.
Note: The content in this article pertains to ArcView GIS versions 3.0, 3.0a, 3.0b, 31., 3.2, and 3.2a only. Later versions of ArcGIS may contain different functionality, as well as different names and locations for menus, commands and geoprocessing tools.
(([LatDMS].AsString.Left(2).AsNumber) + ([LatDMS].AsString.Middle(2,2).AsNumber / 60) + ([LatDMS].AsString.Right(5).AsNumber / 3600))
(([LongDMS].AsString.Left(2).AsNumber) + ([LongDMS].AsString.Middle(2,2).AsNumber / 60) + ([LongDMS].AsString.Right(5).AsNumber / 3600))
Note: On steps 5 and 9, replace the example field names with the actual field names in your table. Don't press the Enter key to move to the next line; allow the expression to wrap automatically.