I think you should have a look at the basic of unreal first, cause storing variable and changing the value is one of the most common thing.
Here a introduction to blueprint:
So first theres no specific way of doing what you asking for, I can think about 3-4 way of doing this quickly. First way would be to simply increase by 10 the variable «ammos» store in your «Game Instance» object, which is a persistent object. That mean you can easily access this from any level OR blueprint.
Because I got the feeling you are really new to unreal, here a video that explain how to use «Game Instance».Hope it help!