DutchMunin or Muninn
Why “Muninn” is correct in this instance, but Munin is too :)
Both are correct, and you will meet both. Muninn is the original Old Norse, in the nominative — the form the raven takes when he is the subject of the sentence, the one doing the flying.
Old Norse declines its names and English does not. In the accusative he is Munin, without the second n. The stanza on Muninn’s front page happens to contain both, six lines apart: Huginn ok Muninn fljúga hverjan dag — they are flying, so they are the subject — and then þó sjámk meirr um Munin, where he is the object of the worry rather than the one doing anything with it.
Translators into English had to pick one form and keep it, and they mostly kept the oblique one. That is why Hugin and Munin is the pair you usually meet in English.
The app is called Muninn because that is his name when he is doing something.
If you would rather hear it explained than read it, there is a video by the Old Norse GOAT, Jackson Crawford.