HOW TO UPGRADE FROM PS4 TO PS5

Ive tried all day and can't suss it out!

Before buying the game I looked online to see how upgrading from ps4 version to ps5 version was done...and theres a whole multitude of different answers. So even though I normally buy my game online, I decided to go into a shop and ask there...and they had no clue either. I have a PS5 but a lot I know have the PS4. So I could play everybody, I was advised to buy the PS4 pre order disc version, install that on the PS5 as a PS4 game, then upgrade once it was installed. However, having done so...there is no button to upgrade, although there is an option to download the PS5 beta? Its definitely installed the PS4 version as I've played other PS4 users today. But having searched for answers...nobody seems to know. Hoping someone from EA will add to this query because it seems like a lot of people are asking the same question and its not being answered. On top of that...I was quite excited because I was told pre ordering early would enable me to receive a one size fits all free T-shirt...the game store were confused as to why they'd not received a delivery...until they were told last minute it was a virtual free t-shirt! 

So to summarise the question, if you have a PS5 with a PS4 disc...how do you upgrade to PS5 version?