Updating tables in SQL Server 2000

Could someone please help with converting the following access 2000 query to a working version of sql 2000:

UPDATE [RXA Claims] SET [RXA Claims].[Fill date] = Mid([Fill Date],5,2) & “/” & Right([Fill Date],2) & “/” & left([Fill Date],4);

The mid function doesn’t work in sql server talk and I need to convert the following text to text resembling a date: 19990701.

Any help would be greatly appreciated.

Thanks