You don't want to clear storage while you still have receipts on the tablet that haven't been submitted yet or you will have to re-enter the receipts.
Once all your receipts have been submitted for the day, you can clear the storage.
To clear the data:
On the home screen your tablet, you should see an Icon called "Settings". Click this icon.
Look for a tab called "Application Manager". Click this.
You should see all your apps on the right. Click the Field Link icon.
Click "Force Stop", then OK.
Click "Clear Data", then OK.
Then click the home screen icon at the bottom left of the tablet to return to the home screen.
You will need to re-register your tablet and re-enter your tax tables.