string replace() function perfectly solves this problem:
string.replace(s, old, new[, maxreplace])
Return a copy of string s with all occurrences of substring old replaced by new. If the optional argument maxreplace is given, the first maxreplace occurrences are replaced.
>>> u'longlongTESTstringTEST'.replace('TEST', '?', 1)
u'longlong?stringTEST'