how i can Clear all balances while retaining customers’ names, suppliers etc.
Use the Batch Delete function. Read the Guide: https://www.manager.io/guides/16464. The Guide discusses the fact that Manager will not let you delete transactions referenced by other transactions. So, for example, you cannot delete a sales invoice until you have deleted any receipt against it. This could be a very tedious operation, depending on how long you have been using Manager.