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
instead most keep their old records and use Manager going forward by using
If your requirements are relatively simple importing the bank statements from the last 1-2 yrs may achieve what you want.