I got the answer and the solution on Ubercart's forum.
Quoting the user longwave:
"In Drupal 6/Ubercart 2 go to
/admin/store/settings/payment/edit/gateways and expand PayPal Website
Payments Pro then Express Checkout.
In Drupal 7/Ubercart 3 go to
/admin/store/settings/payment/method/paypal_ec and look towards the
bottom of the page."
I'm using D6.
Then I mentioned that I see the following message on that page:
"You must enable the Credit Card module to use PayPal Website Payments
... and that I didn't want to enable Credit card module while I don't need it. He said I have to, if I would like to reach "Review payment" settings:
"The settings are hidden here, this is a known bug. Enable the credit
card module; you don't have to use it - or you should be able to
disable it again after changing the Express Checkout settings."
That's true, everything related to "Review payment" form could be set there!
I made a screenshot for others who are also struggling with this problem:
Problem solved. No more unnecessary questions before purchasing. :)