Need help for list of list of tuple !

[webdef]

Quite newbie, I need help to search and modify some elements of a tuple (Python 2.7).
For example, how to replace "aaa" in first element into "bbb" ?

list=[('aaastring',245878L,2475L,'anotherstring', ...),
      ('bbbstring',7894578L,456897L,'yetanother',...),
      ('aaaform',2445L,325478L,'dummy',...),
      (...)
      ]