Singing in a soft dynamic in our extreme vocal ranges is a combination of good vocal technique and breath support.
An example exercise for improving range could be starting with a 5-4-3-2-1 descending pattern on the middle range and working upwards. The important part is to remember how relaxed, easy, and open the middle range is and try to transfer as much of that feeling as you can as you ascend.
There will always be tension in the higher range so be mindful of where that tension is and use it to your advantage. A quick google search can tell you about laryngeal position while singing higher. Most vocalists advocate for a lower larynx position.
Once the technique is there then focus on using your breath to support the dynamic. Practice soft singing in the middle range. Depending on the sound you want will depending on the resonance you are going for. Raspy, strident, dark and so on. I would focus on a warm and resonant sound as singing in that manner will help the voice to stay healthy.