In order to mass edit or add data to assets you have 2 options:
- Use the mass edit button in the left-hand pane on the assets overview page.
- Write a database script to populate the custom fields.
I would recommend to create a backup of your database before performing any modifications. Backup instructions can be found in this knowledgebase article: https://www.lansweeper.com/kb/94/backing-up-your-installation.html