By far the easiest option is to use the Forgot Password link to reset it.
You just have to enter either your username or admin email address and it'll email you a link to reset it:
If you've forgotten your admin username and/or password, you can find it inside the database.
Locate Admin Username / Email In phpMyAdmin
You can access it by going to cPanel -> phpMyAdmin:
Once inside, select the name of your WHMCS database on the left menu, and then locate the tbladmins and click Browse:
You will then see all the admin users you have setup and can easily identify the username and email address:
You'll notice a string under password, but this is your password encrypted. It will not work if you type this string into the WHMCS admin!
You can then take either of these and enter them into the Forgot Your Password field in WHMCS.
Reset Password In phpMyAdmin
If all of the above is not an option, which I have seen before, you can do the reset right inside of phpMyAdmin.
To do so, you'll want to click the Edit button beside the admin user:
You then want to click MD5 under "function" beside password, enter your new password in the value field (erase the current value) and also clear the current password hash:
Scroll down to the bottom and click Go and you're all set!
This is my least favorable method, but it can help you in a desperate situation. Your best bet though is to use the automatic password reset feature.
Contact WHMCS Support
If you're in a real pickle, remember WHMCS also provides FULL support for our resellers.
They have a special ticket desk setup so you don't have to login.
Step By Step Tutorial
I filmed a tutorial where I walk you though all the steps listed above to help you get reset:
If you have any questions, please feel free to post below!