My business is established since Oct 2022. . QB's Bank a/c which linked to my bank, it can only show opening balance from Mar 2022. How can I get them all pls?

I appreciate you sharing your concern here in the Community, @Sweet Somethings. 

Once you have connected your account to Open Banking. QuickBooks depend on the transactions from your bank, some financial institutions allow you to download the last 90 days of your bank entries and have them visible on your banking feeds. 

If you need to transfer all the previous files, we need to import the banking transactions. In this case, we must select the file date for this process's file upload.

 You can follow the steps below: 

1. From the Banking tab, go to the Banking section.
2. Select the Link account ▼ dropdown and then Upload from the file.
3. Select files and then choose the file you downloaded from your bank. Then select Continue in the QuickBooks account ▼ dropdown.
4. Select the account you want to upload the transactions. Then select Continue.
5. Follow the onscreen steps to match the columns on the file with the correct fields in QuickBooks. Then select Continue.
6. Select the transactions you would like to import. Then select Continue.
7. Select Yes.
8. When you accept your transactions, select Done

For additional insights, let me show you a glimpse of this article that will guide you on uploading your banking transaction manually.

Moreover, I've added this article to help you review and ensure your bank and credit card transactions are always matched: Reconcile an account in QuickBooks Online.

Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.