Put this

<?php
define('PHPWG_ROOT_PATH','/var/www/html/piwigo/');
include_once(PHPWG_ROOT_PATH.'include/config_default.inc.php');
include_once(PHPWG_ROOT_PATH.'local/config/config.inc.php');
include_once(PHPWG_ROOT_PATH.'include/functions.inc.php');
if (isset($conf['password_hash']) && !empty($conf['password_hash']) && function_exists($conf['password_hash']))
  echo $conf['password_hash']('yournewpassword') . "\n";
else
  echo "\$conf['password_hash'] not set or function not available: '" . $conf['password_hash'] . "'\n";
?>

(of course adapt the PHPWG_ROOT_PATH setting and yournewpassword ...) in a file, either to be invoked as
php filename.php
from the (ssh) command line, or reachable in your web server's document root, for example /var/www/html/mypassword.php
In the latter case point your browser to it, it'll display some $P$... string, copy that. Remove the file.
Open your Piwigo database using phpMyAdmin or whatever tool you have, in table piwigo_users locate your admin user (probably first record), edit that and paste the hash string into the password field.
Close database, remember password ;)

Note that putting the password into the PHP snippet file may be unsafe if malicious actors have access on file level, you may want to use a form then instead.

Also, this uses the configured $conf['password_hash'] method that is default defined to
$conf['password_hash'] = 'pwg_password_hash';
If you changed that in your Piwigo config it should still work fine. If you did something else then adapt.