You do need to cancel Flex, then you have to confirm it at a ticket machine. You can, and need to travel with a balance after this.

Those. If I want to travel during off-peak hours, is it only with card debiting? Or is it possible to pay in advance?

You can buy a subscription, Dal Voordeel. You do need tot pay €61,20 in advance for a year with 40 percent reduction with a check-in during off-peak hours. You need a balance that has to be placed on your card in advance as well.

It can be placed on your card automatically when your balance is not sufficient.

BUT you have to pay this 61€ before-to get the discount for 1 whole year-and you cannot terminate before. The name is DalVoordeel. If you stay here for not too long then it might be too expensive for what you get.

Depending on how many trips by train you make and how long, there may be other subscriptions as better deal.

That is incorrect; non-Flex Dal Voordeel can be ended every month after the first month.

If you want to pay for your trips in advance, buy e-tickets. They will not give you any discount, though.

Please tell me, how can I pre-load 61 euros on the card if this function is not available in my tariff now?

Apparently, you have NS Flex for which preloading balance is not possible. Travel costs are billed monthly.

You can order Dal Voordeel (40% off-peak reduction for trains) as a subscription, this costs € 61,20 for a year. Once you have that, you can load balance on your card at a machine.

https://www.ns.nl/en/nsflex/webshop#/bestelling/producten/2

You can order it here. Take ‘On balance’. You cannot pay with your balance on your card. Paying for a subscription and traveling on balance are different things.