If you want to change the default application for a certain file type (for example .pdf) you can:

1. Click start

2. Type "Default apps"

3. Click  "Default apps" 

4. In "Default apps" scroll down to "Choose default apps by file type"

5. In "Choose default apps by file type" scroll down to the file type you want to change (for example .pdf)  

6. Set it to the correct default app (for example Acrobat DC).