How to edit particular value in session table?

[eluser]peekk[/eluser]
Hello,
when I type

Code:

print_r($this->session->all_userdata());

it shows

Code:

[user_data] => [search_conditions] => Array ( [aaa] => value1, [bbb] => value2 ) )

I want to change only the value of 'aaa' index in search_conditions table (example: 'value1' into 'changed_value1' - how to do it?