You're missing the apostrophes. But a better idea is to use a parameterized statement.
string st = "update Mess_Plans set Amount=@amt where PlanName=@plan";
OleDbCommand cmd = new OleDbCommand(st, con);
cmd.Parameters.AddWithValue ( "@amt" , amt ) ;
cmd.Parameters.AddWithValue ( "@plan" , plan ) ;
I also hope you are not putting your Data Access Code directly in your UI code.