You can achieve this using simple SQL update statment.

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 employee by 10 percent. For this I can wrire a query as below

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

This will increase all employee salary 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