Take a look at this, hope this will help.
int convertFive(int n)
{
    int count = 1,num=0,rem,res;
        while(n != 0) // checking whether number is not equals to zero
        {
            res=n/10; //breaking the last digit
            rem=n%10; // finding the last digit
            if(rem==0) //replacing it with 5
            {
            num=num+count*5; // building the new number
            }
            else num = num + rem*count; //if it is not 0, building the new number
            count = count*10; // increasing the place value for next digit.
            n=n/10;
        }
        return (num);
//Your code here
}
Hope this helps.