On the iPad Till app itself, the transaction data stays on the iPad for only 2 or 7 days depending on your posBoss app settings.
Reprint the transaction from the iPad
You can locate previous transactions under Settings > History. You can then select from orders, transactions or cash ups. Select the transaction you're after and press the Reprint button.
Account payments can be reprinted here too, but have no itemised list and will appear blank on the screen.
Locate older transactions
To find a transaction further back than this, you can do so for any day in the past on the management site, under Financials > Transaction history. Or for account payments head to Financials > Customer Accounts
posBoss History Settings
As mentioned above, you can change your posBoss iPad settings to display 2 or 7 days data by going to Settings on your iPad home screen and scrolling down the left hand panel until you find posBoss.
Select it, then press Retain history on cash up to change between 2 and 7 days: