This page explains how to find and activate Hebra Tower in Hyrule in Breath of the Wild.
How to Find the Tower
Bring a heat source with you, such as a flame weapon, or some wood and flint.
You will also need protection from the cold, so be sure to either buy the Snowquill armor from Rito Village, or bring food or elixirs that increase cold resistance.
From Akh Va'quot Shrine, go to the stable east of the village and take out a horse if you have one. Then go east on the road from the stable. The road goes past Hebra Tower.
How to Climb the Tower
The tower is surrounded by large chunks of ice that you can't climb. Use either a flame weapon or a campfire (hit flint with a metal weapon near a bundle of wood) to melt the ice. The tower has many platforms, so after you get rid of some of the ice, it is easy to climb.