EQ is only available if your headphones support it. Check your product documentation to be sure.
If supported, use the Sennheiser Smart Control app to set up the internal EQ.
After you finish, the EQ settings are saved inside the headphones.
This makes your personalised sound stay the same with any music source (phone, computer, etc.).
Tip:
You can re-open the app later to adjust the EQ again.