'continue' the 'for' loop to the previous element

Here when the corresponding value of i is equal to c the element will change to your request and go back one step, reprinting b and abc, and finally d:

foo = ["a", "b", "c", "d"]
i = 0
while i < len(foo):
    if foo[i] == "c":
        foo[i] = "abc"
        i -= 1
        continue
    print(foo[i])
    i += 1

In a for loop you cannot change the iterator. Use a while loop instead:

foo = ["a", "b", "c", "d"]
i = 0
while i < len(foo):
    if foo[i] == "c":
        foo[foo.index(foo[i])] = "abc"
        i -= 1
        continue
    print(foo[i])
    i += 1