Hi,

To delete the journal entries you can use the module account_cancel .  This will help you to delete the journal entries.

For the testing purpose create db and do the operations there in  it , once after everything is done and tested, do the same set up in the new db, and drop the test db.

Have a look at this link too : https://www.odoo.com/zh_CN/forum/help-1/question/reset-database-120267

For Latest Version, use this free app:  https://apps.odoo.com/apps/modules/16.0/om_data_remove/

Thanks