You have the right to paint and wallpaper the apartment at your own expense. Keep in mind that the work must be done professionally and that the changes must not be too drastic. You may then need to restore the affected surfaces when you move out.
You may not, without permission, alter the apartment’s floor plan by removing or adding walls, changing or creating new openings in walls, or removing or moving permanently installed fixtures in the apartment. Always contact your building manager and consult with them before beginning any work!