How to organize (update/delete) data in databases (general question)

Hello, I'm programming with HTML, PHP, JS, but I'm not really familar with databases. (mySQL in this case)

E.g. I have written an editor for textemplates. Now, I have created some dummy templates, however I want to work with my real templates from work, (JSON export files). Can I just overwrite the database (import the files, make a backup before), or do I create a new database? How do you usually hande this? Thanks!

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