A stupid but straghtforward ans:

# get the md5 pass for ur admin account
# make a small php script
# filename: printmd5password.php
<?php
echo md5("newPassword");
?>

# at your shell, run the script.
$ php -f printmd5password.php
# the script will print our the 'newPassword' with md5 hashed

# use MySQL to update the 'admin' password
$ MySQL -u root -p
< enter your mysql's root acct password

# you have login the MySQL client.
# select the database which is used by ur XOOPS installation
> use xoops
# run a SQL statment to update/overwrite ur admin password
> update xoops_users set pass='XBADWF#$#@' where uid='1';

# note, the pass='' should match what output from the php
# script above.
# and uid=1 means the first record in the table xoops_users
# which is usually the admin account ur've created