Overwriting a cookie in request header

I have an Apache module in which I am trying to overwrite the value of a cookie. I have tried different methods of the APR::Table without success.
Here are the approaches that I have tried:

[1] $r->headers_out->set("Set-Cookie", $cookie);

Here, $cookie has the name=value pair with the name of the cookie that needs to be overwritten. The outcome is 2 cookies with the same name.

[2] $cookie = $r->headers_in->{Cookie};
<search-and-replace the cookie value in $cookie>
$r->headers_out->{Cookie}=$cookie;

Does not do anything to the existing cookie ... does not even add a new cookie.

[3] $cookie = $r->headers_in->{Cookie};
@cookies = split(/;/,$cookie);
$r->headers_out->clear();
< add cookies one-by-one replacing the value of the cookie in question using  $r->headers_out->set("Set-Cookie", $cookie); >

The web application does not work ... seems like clearing the header creates problems.

Any pointers would be really helpful.

Thanks,
Ritu