I wouldn't attempt this if you don't have an understanding of the database structure. One mistake and you could ruin the install. Personally I would find someone to do it for me (place an ad in classifieds).
In order to do it you would need to know how to access your mysql database directly through the CLI or through a program like phpMyAdmin.
the CLI is accessed from a root login and type 'mysql -u username -p'
then after you are logged in type 'USE databasename' of course replacing databasename with the name of your sugarcrm database.
then run this code (I do not guarentee this AT ALL, do this slowly and check each table to make sure it's not data you want to keep, again I would get an expert to backup your system and clear it correctly)
Code:
TRUNCATE `accounts`;
TRUNCATE `accounts_audit`;
TRUNCATE `accounts_bugs`;
TRUNCATE `accounts_cases`;
TRUNCATE `accounts_contacts`;
TRUNCATE `accounts_opportunities`;
TRUNCATE `bugs`;
TRUNCATE `bugs_audit`;
TRUNCATE `calls`;
TRUNCATE `calls_contacts`;
TRUNCATE `calls_users`;
TRUNCATE `campaigns`;
TRUNCATE `campaigns_audit`;
TRUNCATE `campaign_log`;
TRUNCATE `campaign_trkrs`;
TRUNCATE `cases`;
TRUNCATE `cases_audit`;
TRUNCATE `cases_bugs`;
TRUNCATE `contacts`;
TRUNCATE `contacts_audit`;
TRUNCATE `contacts_bugs`;
TRUNCATE `contacts_cases`;
TRUNCATE `contacts_users`;
TRUNCATE `documents`;
TRUNCATE `document_revisions`;
TRUNCATE `emailman`;
TRUNCATE `emails`;
TRUNCATE `emails_accounts`;
TRUNCATE `emails_bugs`;
TRUNCATE `emails_cases`;
TRUNCATE `emails_contacts`;
TRUNCATE `emails_leads`;
TRUNCATE `emails_opportunities`;
TRUNCATE `emails_projects`;
TRUNCATE `emails_project_tasks`;
TRUNCATE `emails_users`;
TRUNCATE `email_marketing`;
TRUNCATE `email_marketing_prospect_lists`;
TRUNCATE `email_templates`;
TRUNCATE `feeds`;
TRUNCATE `inbound_email`;
TRUNCATE `inbound_email_autoreply`;
TRUNCATE `leads`;
TRUNCATE `leads_audit`;
TRUNCATE `meetings`;
TRUNCATE `meetings_contacts`;
TRUNCATE `meetings_users`;
TRUNCATE `notes`;
TRUNCATE `opportunities`;
TRUNCATE `opportunities_audit`;
TRUNCATE `opportunities_contacts`;
TRUNCATE `project`;
TRUNCATE `project_relation`;
TRUNCATE `project_task`;
TRUNCATE `project_task_audit`;
TRUNCATE `prospects`;
TRUNCATE `prospect_lists`;
TRUNCATE `prospect_lists_prospects`;
TRUNCATE `prospect_list_campaigns`;
TRUNCATE `tasks`;
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