Speedier , cleaner T-SQL code

I've got a table which i want to populate with information from two sources.
All except one column come from one source and i am using a join to determine which rows should be modified with information in the last column.
I've got this procedure which takes too long to run plus i'm using an UPDATE
statment and i shouldn't because it's supposed to be data warehouse.  
I want some code that will let me put a value in the waitlist_position colum without using UPDATE, perhaps somehow using INSERTs instead and if possible execute a bit faster.

DECLARE @number_waitlisted int
DECLARE @rowcount int
DECLARE @filexists int
DECLARE @filecopied int
EXEC @filexists = master..xp_cmdshell "dir d:\inetpub\ftproot\dwfact"
 IF @filexists = 0
   BEGIN
     EXEC @filecopied = master..xp_cmdshell "copy d:\inetpub\ftproot\dwfact d:\mssql\ftpdir\units"
     IF @filecopied = 0
       BEGIN
          CREATE TABLE #bcpunits(bcpoutput varchar(255) NULL)
          CREATE TABLE #units
          (
         SECTION_KEY varchar (15) NOT NULL ,
         STU_KEY int NOT NULL ,
         TERM varchar (5) NOT NULL ,
         .
         .
         FROW_COUNT int NOT NULL,
                  WAITLIST_POSITION varchar (2) NULL
            )
       

       
        INSERT INTO #bcpunits
          EXEC master..xp_cmdshell 'bcp Tempdb..##units in d:\mssql\ftpdir\units -f d:\mssql\ftpdir\units.fmt -Usa -Psss'

           
          DELETE FROM units
          WHERE term IN
          (SELECT term
           FROM#units)

     
         INSERT units
            EXECUTE ( "SELECT
              SECTION_KEY ,
              STU_KEY ,
            TERM  ,  
                      ENRL_WAIT  ,
            .
                      .
            OPNU_UNITS,
            FROW_COUNT,
                      ' '
          FROM ##units" )
           
     
       
          EXEC @filexists = master..xp_cmdshell "dir d:\inetpub\ftproot\waitlist"
          IF @filexists = 0
             BEGIN
               CREATE TABLE #bcpwaitlist(bcpoutput varchar (255) NULL)
               CREATE TABLE ##waitlist
                (
                 numberwaitlisted     char (2) NOT NULL,
                 waitlist_position    char (2) NOT NULL,
                 student_id           char (9) NOT NULL,    
                 pin                  char (4) NOT NULL,
                 name                 char (32) NOT NULL,
                 course_section       char (11) NOT NULL
                )    


              INSERT INTO #bcpwaitlist  
               EXEC master..xp_cmdshell 'bcp Tempdb..##waitlist in d:\mssql\ftpdir\waitlist -f d:\mssql\ftpdir\waitlist.fmt -Usa -Pssssr'
           
              CREATE INDEX idx_temp_units ON  Tempdb..##units (section_key, stu_key)
              CREATE INDEX idx_waitlist ON  Tempdb..##waitlist (course_section, student_id)
         

              SELECT @number_waitlisted = (SELECT COUNT(*) FROM #waitlist)

             

              IF @number_waitlisted > 0
                 BEGIN
                    UPDATE#units
                    SET ##units.waitlist_position =
                   ( select #waitlist.waitlist_position
                     FROM #waitlist, student
                     WHERE #units.stu_key = student.stu_key
                     AND student.stu_id =#waitlist.student_id
                     AND#waitlist.course_section = substring(#units.section_key, 6, 15))
                 END
           END
        END
     .
     .
     .