That depend what you are intending to use Manager to do and what outputs you want from Manager ie your business goals and essential requirements. I suggest starting by setting a dummy business, loading the Philippines localisation & experimenting with account structures, data entry and reporting options. You will also need to read several of the guides https://www.manager.io/guides/ and enable tabs ( Customize a business ) only as required.
It is possible but beyond the abilities of many users, see How to batch import in manager - #11 by Patch
instead most keep their old records and use Manager going forward by using
- Starting Balances: Set start date | Enter starting balances
If your requirements are relatively simple importing the bank statements from the last 1-2 yrs may achieve what you want.