Wow… That’s pretty much hidden now (at least during transition from the old layout to the current one). Here’s a path I used to dig up the option from redi’s screenshots:
Settings (Gear icon next to the “log out” in the top right corner) > Payment Settings > Manage pre-approved payments > manage pre-approved payments (yes, again - this time the link was in the left column and opened new tab with the old layout) > Set Available Funding Sources > Convertion Options (a link next to the card you want to change). Voila! 
They really made the effort not to make it easy to most users… 
But at least it seems it should work with my Revolut.