set @anyVariableName:=yourValue;
update yourTableName set yourColumnName=yourColumnName+@yourVariableName;

mysql> create table DemoTable
-> (
-> Number int
-> );
Query OK, 0 rows affected (0.68 sec)

mysql> insert into DemoTable values(10);
Query OK, 1 row affected (0.18 sec)

mysql> insert into DemoTable values(20);
Query OK, 1 row affected (0.11 sec)

mysql> insert into DemoTable values(40);
Query OK, 1 row affected (0.15 sec)

mysql> insert into DemoTable values(60);
Query OK, 1 row affected (0.13 sec)

mysql> select *from DemoTable;

+--------+
| Number |
+--------+
| 10     |
| 20     |
| 40     |
| 60     |
+--------+
4 rows in set (0.00 sec)

mysql> set @intValue:=100;
Query OK, 0 rows affected (0.00 sec)

mysql> update DemoTable set Number=Number+@intValue;
Query OK, 4 rows affected (0.15 sec)
Rows matched: 4 Changed: 4 Warnings: 0

mysql> select *from DemoTable;

+--------+
| Number |
+--------+
| 110    |
| 120    |
| 140    |
| 160    |
+--------+
4 rows in set (0.00 sec)