umd = New OleDbCommand()
umd.CommandText = "UPDATE STUDENT SET roll = ' " + TextBox1.Text + " ' , name = ' " + TextBox2.Text + " ', marks = ' " + TextBox3.Text + " ' " & " WHERE roll = ' " + TextBox1.Text + " ' "
umd.Connection = con
da.UpdateCommand = umd

da.Fill(ds, "student")

I am using the above code for updating a row in database (MS access) but its not working. Please help me in solving this. Thanx.

Which part is not working? What error do you get?

At least using TextBox1.Text in both SET and WHERE seems a bit suspicious. If roll exists, you update roll-field with the same value it already has. If roll does not exists, then your update fails.

Also, open connection before you assign it to command:
umd.Connection = con

It is still the same.......I have removed the roll field from update and also I have opened connection before assigning it to a command..........

is there any condition to execute the block of code ?

Do you get an error message? Or does your code "work" i.e. no error message, but the database will not be updated?