How to add data to existing child in firebase database?

  android, android-layout, android-studio, firebase, java

In my firebase database,i have users resister data. I want to add images in the existing data of the users. So the process will be from one activity i want to add other details in the database in user child and from another activity i want to store image link to the same child(user) because the image will store in firestore. If some one suggest me how to achieve it, i will be very thankful.

Source: Android Questions