You can achieve this using a simple SQL update statement.

Let us consider a table called employee. Here I got four rows.

IDE          Name_NM               Salary_SA

1                  Nayani                   1000
2                  Kapil                      1000
3                  Sundhar              10000
4                  Devis                   20000

I want to increase the salary of employees by 10 percent. For this, I can write a query as below

UPDATE Employee SET Salary= (Select Salary+ (Select Salary*0.1))

This will increase all employee salaries by 10 percent.

IDE           Name_NM                  Salary_SA

1              Nayani                1100
2              Kapil                   1100
3              Sundhar             11000
4              Devis                 22000

For suppose I want to increase a particular employee salary by 20 percent I can write a query as

UPDATE Employee SET Salary= (Select Salary+ (Select Salary*0.1)) Where ID = 2

Hey, try this function it worked for me. 

SQL> create or replace function calculate_emp_salary_increment
2 (incpercent in number,
3 salary in number
4 )
5 return number
6 as
7 begin
8 return salary * incpercent/100;
9 end;
10 /

Function created.

SQL> select emp_id, emp_name, salary, calculate_emp_salary_increment(20, salary) INCREMENT
2 from employee;

EMP_ID EMP_NAME SALARY INCREMENT
---------- ---------- ---------- ----------
1234   MIKE   2450               245
1235   Peter   5000               500
1236   JOHN  1300                130