Log into your hosting control panel and go to the databases section.  Find the link to start phpMyAdmin so you can manage the database.

Next, select the database that is serving your Joomla site.

On the left side of the screen is a list of all of the tables that make up your Joomla database.  Find the jos_users table and click on that.

Next, click on "Browse" at the top of the screen so you can see the records in the jos_users table.

Now, the first record is generally the "admin" record created when Joomla is first set up.  Click on the icon at the beginning of the line so we can edit the "admin" record:

How, we will change the data type to "MD5" and enter a new password in the field titled "password."

The final step is to "save" these changes.  Do this by clicking the "Go" button in the lower right corner of the screen.

Once that is done, you have now changed the "admin" password to whatever you entered.  If you used something "simple" like "password" be sure to log in to your Joomla back end and change it to something more secure.  You may want to change the login name from "admin" to something else as well to give you an additional layer of security