I have recently brought a Lenovo yoga tab 3 but when I opened the box it didn't look very good and wasn't an 8 inch screen as advertised?

I now can't reseal it as the stickers on the case say void what do I do?

Take it back to the shop.

If the product was not what was advertised then the retailer has a legal obligation to rectify that regardless of what those stickers say. Take it back and demand a refund or an exchange for a product that was specified. If you bought it online return it and charge back the card payment.