HI,
You can't decode the password, except with brute force tools. DW uses a one way hashing function, not a two way encryption function. That said, you can reset a password. See for example andi's post in thread
2778, or as dominik states in the same thread
https://www.dokuwiki.org/faq:forgotten_password .
Please also note that the "problem" might not be what you think it is. Options set in
conf/local.protected.php cannot be changed via the admin panel, even by an admin. You should check that file first before resetting password. Manual edit of the file is required. This is a security feature.