In short - you can't - without spending a lot of time learning the data model, ensuring you delete from related tables, resetting sequences, etc.  Even then you could miss something and put your database into an inconsistent state.

Modules are portable and can be installed on a fresh, empty database.  Other customizations you should export and import into a fresh, empty database.

This is really the same question as:

https://www.odoo.com/forum/help-1/question/how-to-remove-demo-data-from-an-already-live-and-running-application-22683

As Ray pointed out, there's no easy way to 'clean' a database. It's possible to delete/reverse some transactions, but that may have subtle effects that do not show themselves until later down the line.

The best advice is to set up a 'baseline' database, which contains configuration/settings, but no transaction data. This would ensure that you always have a 'clean' state to revert to should something go wrong with your live database. I'd also suggest configuring daily backups, as this saves a LOT of headache should something go wrong.

Backups module: https://apps.openerp.com/apps/modules/8.0/auto_backup/