facing sql query problem

i have a table “emp”
it have four column – id,name,salary,grade
values is-1st row- 1,aa,10000,a
2nd row- 2,bb,5000,b
3rd row- 3,cc,2000,c
now i want to add the value in salary column on the basis of grade
like – add 5000 for grade a
add 3000 for grade b
add 2000 for grade c
please help me..
thank you!