To level up any models you own. You will need to collect Model Icons of the right type for your model.
Each model will have one of the following Icons associated with them:
- Active - Lotus Flower Icon (orange)
- Alternative - Skull Icon (purple)
- Glamorous - Lips Icon (red/pink)
- Professional - Book Icon (blue)
- Studio - Studio Light (green)
Once you have enough icons to level up go to the Models and Studios menu (The figure and spotlight on the main feed). From here select the model you want to level up and you'll be able to spend icons to advance their level and unlock new content from that level.