Requirements: Photoshop or Gimp (with basic understanding of layer masks)

Establish a Solid Base:
Create two duplicate layers of your hair image.
Apply a layer mask to the first duplicated layer.
Use a mixer brush on the first layer to blend the surrounding area into the hair, eliminating any visible edges. Temporarily hide this layer.\
Enhance Hair Flow:
On the second duplicated layer, use the mixer brush to follow the natural flow of the hair.
Focus less on the edges, as they will be refined using brushes later.
Drawing in Hair:
Always create a new layer (not a duplicate) when adding new strands of hair.
Start by drawing the back of the hair first, then work your way forward.
When you work this way, the layers will be automatically organized based on the depth of field and proximity to the camera. The closest hair to the camera should be the topmost layer, while out-of-focus or background hair should be just above the second duplicated layer.
Utilizing Brushes:
Included below are links to a few free brushes that can be used, although creating your own is recommended for optimal results.
Apply the brushes to areas that appear pixelated or require improvement.
Adjust the opacity of the second duplicated layer to reveal the original screenshot, helping you identify areas that need attention.
Vary the colour in your brush settings, or frequently use the eyedropper tool to avoid repetitive colours.
Removing Unwanted Hair:
The first duplicated layer can be used to remove entire sections of hair from the original screenshot if desired.
Additional Resources:
Refer to the linked video for comprehensive instructions on applying and transforming brushes to your image.
If you're not skilled with the pen tool, using transform and warp options can save considerable time.
By following these steps, you'll be able to achieve impressive, photo-realistic hair in Cyberpunk 2077. Enjoy!

Free brushes: One / Two / Three / Four / Five / Six / Seven 

Video guide: