After you have done your tests on your new WHMCS installation, you may want to remove all dummy clients, invoices, products, etc
You can TRUNCATE the following database tables:
TRUNCATE `tblaccounts` ;
TRUNCATE `tblactivitylog` ;
TRUNCATE `tbladminlog` ;
TRUNCATE `tblaffiliates` ;
TRUNCATE `tblaffiliatesaccounts` ;
TRUNCATE `tblaffiliateshistory` ;
TRUNCATE `tblaffiliatespending` ;
TRUNCATE `tblbrowserlinks` ;
TRUNCATE `tblcancelrequests` ;
TRUNCATE `tblclients` ;
TRUNCATE `tblcontacts` ;
TRUNCATE `tblcredit` ;
TRUNCATE `tblcustomfieldsvalues` ;
TRUNCATE `tbldomains` ;
TRUNCATE `tbldomainsadditionalfields` ;
TRUNCATE `tblemails` ;
TRUNCATE `tblgatewaylog` ;
TRUNCATE `tblhosting` ;
TRUNCATE `tblhostingaddons` ;
TRUNCATE `tblhostingconfigoptions` ;
TRUNCATE `tblinvoiceitems` ;
TRUNCATE `tblinvoices` ;
TRUNCATE `tblnotes` ;
TRUNCATE `tblorders` ;
TRUNCATE `tblticketlog` ;
TRUNCATE `tblticketnotes` ;
TRUNCATE `tblticketreplies` ;
TRUNCATE `tbltickets` ;
TRUNCATE `tblupgrades` ;
TRUNCATE `tblwhoislog` ;
Use phpMyAdmin in your cPanel to TRUNCATE these tables.