Good evening Mmifsud. Welcome to the NS Community.
That's sadly not possible. You can't activate your product before the date that you've ordered it to start. We also can't adjust this for you.
Welcome to the NS Community!
Unfortunately, the starting date for Weekend Vrij cannot be changed, even if you received the card earlier.
You can use the card to travel without loading balance, but will get no discount until July 17.
It might be possible to use your withdrawal right https://www.ns.nl/en/customer-service/termination-cancellation/cooling-off-period.html to cancel the Weekend Vrij order.
You will then need to remove NS Flex from the card at a machine (station or supermarket) and then order NS Flex again (the next day), with the starting date as ‘immediately’ (active after activation at a machine).