How to complete a test item in Java?

Question

Java

I did almost all the test task in Java, wrote the rest api on Spring boot with h2 base, but could not figure out how to implement the following point:
• All system data must be stored in a file. There are no restrictions on the data storage format.
• [Unclear:] The user should be able to specify which data file to use.

As I understand it, we are talking about jdbc: h2: file:. If so, how can the user change application.properties? And if not, then what?

h2 is optional, not required. Application data is class objects (projects, users, and tasks).