textToWork = u"авпж"
textToWork.replace(u'ж', 'lalala')
print textToWork

Help on built-in function replace:

replace(...)
    S.replace (old, new[, count]) -> string

    Return a copy of string S with all occurrences of substring
    old replaced by new.  If the optional argument count is
    given, only the first count occurrences are replaced.

textToWork = u"авпж"
result = textToWork.replace(u'ж', 'lalala')
print result