Good day, jyothi-c.
I understand that you want to delete all customer's transactions at once.
In QuickBooks Online, we can only delete them individually. Open the transactions, click More at the bottom and select Delete.
If you're planning to start from scratch, you'll want to purge your company. Though, this is only applicable to those less than 30 days old accounts.
Post again here if you have more questions. We'll respond as soon as we can.