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. It can be located under Settings > History. You can then select from orders, transactions or cash ups.
To find a transaction further back, you can do so for any day in the past on the management site, under Financials > Transaction history.
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: