What you are looking at is serialized data– possibly multiple bits of serialized data as @Rarst noted. The Options API will serialize and unserialize automatically so all you need to do is:

$data = get_option('optionkey');
// $data will be unserialized into whatever data type it was originally, object, array, etc
// manipulate your data with ordinary PHP object and array techniques
update_option('optionkey',$data);

Reference

http://codex.wordpress.org/Function_Reference/get_option
http://codex.wordpress.org/Function_Reference/update_option