It's not possible to get there without a 4x4 car, which means that this place usually isn't that touristic. Take the road 32 (road 26 is still partly a gravel road which means that depending from where you are coming road 32 could still be faster) until you reach the gravel road 332, which is only accessible via 4x4. From this fork you need about 15 minutes to drive up to the Haifoss waterfall.
If you don't have a 4x4 you can leave your car at the tiny guest house, which is already on the gravel road, but until the fork Haifoss/Holaskogur you can still use any car and walk from there to the waterfall, which is a hike of around 2 hours.
If you get there please make sure that you take care of yourself and stay away from the cliffs. There is nothing that can save you if you fall down there for over 100 meters, so don't be silly and risk your life for a selfie (I am saying this for a good reason, seeing people standing as far as they can stand on the edge to get the best possible shot of them...).