You have to do this through the WordPress database directly. The most convenient way to manage the database is via the phpMyAdmin tool at your web hosting account.
Once in phpMyAdmin select the WordPress database from the drop-down menu on the left. The page will refresh and the database's tables will be displayed on it. Open the SQL tab (look at the top navigation bar).
In the text field write the following SQL query:
UPDATE `wp_users` SET `user_pass` = MD5('new_password_here') WHERE `wp_users`.`user_login` = "admin_username";
"new_password_here" - replace this with the new password you wish to use.
"admin_username" - replace this with the username the password should be updated for.
Once you are ready, click on the GO button to submit the query. If everything goes fine without errors, you should be able to login to WordPress with the new password.
If you have problems resetting the password yourself, ask your host for assistance. SiteGround provides the best WordPress hosting and such requests are handled with ease.