Python Beginner needs help with for loop

[ty.kresher at gmail.com]

Hi, I'm trying to write a for loop in place of the string
method .replace() to replace a character within a string if it's
found.

So far, I've got

s = raw_input("please enter a string: ")
c = raw_input("please enter a character: ")
b_s = s[::-1] #string backwards

found = False #character's existence within the string

for i in range(0,len(s)):
    if(b_s[i] == c):
        print "the last occurrence of %s is in position: " % (c),
        print (len(s)-i)-1 #extract 1 for correct index
        found = True
        break #run just once to print the last position
if found:
    s2 = s.replace(c,"!")
    print "the new string with the character substituted is: " +
s2

I need to replace s2 = s.replace(c,"!") with a for loop function
somehow.

I don't really see how a for loop would iterate through the string and
replace the character, and would like to see an example if possible.