Help in Complex query

Can you try the below

 
DECLARE @MyVariable int;
Select @MyVariable=MAX(code_t1) from #T1
DECLARE @rownum int=0
DECLARE @id int;
Declare @code_desc varchar(10)='0334'

DECLARE upd_cursor CURSOR 
FOR SELECT code_t1 FROM #t1

OPEN upd_cursor 
FETCH NEXT FROM upd_cursor 
INTO @id
WHILE @@FETCH_STATUS = 0
BEGIN
        set @rownum=@rownum+1
        UPDATE #t1 
        set code_t1 = (@MyVariable)+@rownum,
        code_t1_desc=replace(code_t1_desc,SUBSTRING(code_t1_desc,0,CHARINDEX('_',code_t1_desc)),@code_desc)
        where code_t1=@id
        FETCH NEXT FROM upd_cursor 
        INTO @id
        
END
CLOSE upd_cursor;
DEALLOCATE upd_cursor;

select * from #t1





     

For the first part answer,Please modify the second part accordingly.

--Prashanth