C# SQL update query

Question

 i am using visual studio 2015,

 

update command using sql command

 

string sql = "UPDATE STU set PRESENTEDHOURS=PRESENTEDHOURS + 5,TOTALHOURS=TOTALHOURS + 5 WHERE DEPARTMENT='COMPUTER SCIENCE' AND COURSE='" + comboBox1.Text.ToString() + "' AND YEAR='" + comboBox2.Text.ToString() + "' AND SHIFT='" + comboBox3.Text.ToString() + "'";

 

but updated presented hours and total hours 10 values