string sql = "SELECT 备注,REPLACE([备注],'哈哈','哈哈谢谢')AS 新备注 FROM Sheet1";
            DataSet ds = new DataSet();
            OleDbDataAdapter da = new OleDbDataAdapter(sql, mycon);
            da.Fill(ds, "Sheet1");
            this.dataGridView1.DataSource = ds.Tables[0];

declare @tab table
(
    [备注] NVARCHAR(30)
)
insert into @tab
select 'abc哈哈fghi哈哈3rw'
SELECT [备注],REPLACE([备注],'哈哈','哈哈谢谢') AS 新备注 from @tab
/*
备注                             新备注
------------------------------ --------------------------------
abc哈哈fghi哈哈3rw                 abc哈哈谢谢fghi哈哈谢谢3rw

(1 行受影响)
*/

------解决方案--------------------
你这样试试看
C# code


string sql = "SELECT 备注,left([备注], instr([备注], '哈哈')) + '哈哈谢谢'+ right([备注], len([备注]) - instr([备注], '哈哈') - len('哈哈')) 
 AS 新备注 FROM Sheet1";
DataSet ds = new DataSet();
OleDbDataAdapter da = new OleDbDataAdapter(sql, mycon);
da.Fill(ds, "Sheet1");
this.dataGridView1.DataSource = ds.Tables[0];