The route is very obvious: You walk up where it appears flat and easiest. There is no fix route and there will never be a track, especially in the upper parts: There you have to find out yourself, how to circumvent the crevasses. Once you reach the crater rim, a short passage to the left will carry you to Haakon VII toppen, the highest spot.
This only applies in excellent weather. The slightest question mark in that respect means you have to postpone any activity. Until your time on the island runs out ...
"After the first glass, you see things as you wish they were. After the second, you see things as they are not. Finally, you see things as they really are, which is the most horrible thing in the world."
--Oscar Wilde on Absinthe