Pagination on single portfolio pages

Hi

I use the pagination on my portfolio pages, but the pagination is backwards. How can I edit the code, so when I click on the arrow on the right, it will show the next portfolio page, and not the previous.
Actually it works, but the links which is assigned to the left and right arrows, have to be vice versa.
Example of how it is now: https://scantron.dk/produkter/alpha-central/

Best regards
Nanna