Once we can see mildew it unfortunately is too late to take precautions such as regular ventilating, heating, dehumidifying using an air conditioner (if available).
The fungal organisms and their spores will dwell deep within your wallpaper and possibly even underneath it. Superficially removing them, or treating them with chemicals will only lead to a very temporal relief. They will come back sooner or later.
We therefore have to completely remove all of the affected wallpaper. We may also have to remove the plastering, if affected. We will only find out after the wallpaper was removed.
Before we decorate the wall again we need to make sure all moisture is gone. This needs heating of the room and it needs frequent ventilation. To speed up the drying process there are special dehumidifying devices for rent (note that power consumption will be more expensive than the rental fees).
We then may treat the underground with a special anti-mildew preparation for disinfection before we mount our new wallpaper.