The basic error in your SQL is that the additional conditions are just "and" and not "And where"
The scratch-my-head problem is why the first half of your query uses parameters and the second does not. THIS NEEDS TO BE FIXED!
Part 1 will be to fix your WHERE clauses:
LvMasterCmd.CommandText = "UPDATE LeaveMaster SET From_Date= @From_Date, To_Date=@To_Date, LeaveType=@LeaveType, Days=@Days, LeaveStatus=@LesveType WHERE Sap_No= @SapNo AND MNT = @Mnt And YR = @yr"
And Part 2 will be to add the new parameters:
LvMasterCmd.Parameters.AddWithValue("@From_Date", DtpFrom.Value.ToString)
LvMasterCmd.Parameters.AddWithValue("@To_Date", DtpTo.Value.ToString)
LvMasterCmd.Parameters.AddWithValue("@LeaveType", CmbLeaveType.Text)
LvMasterCmd.Parameters.AddWithValue("@Days", LblDays.Text)
LvMasterCmd.Parameters.AddWithValue("@LeaveType", "Y")
LvMasterCmd.Parameters.AddWithValue("@Sap_No", TxtSapID.Text)
LvMasterCmd.Parameters.AddWithValue("@MNT", CmbMonth.Text )
LvMasterCmd.Parameters.AddWithValue("@YR", CmbYear.Text)