To retrieve your existing iOS Distribution Certificate P12 file:

Note: You must use the Mac computer on which the original P12 file was generated.

1. Click the magnifying-glass icon at the top-right corner of your screen.
    
2. Type Keychain Access into the Spotlight search bar.
    
3. Click Keychain Access. 
   
   
4. Click My Certificates under Category on the left side of the screen.
   The certificates stored on your computer are displayed in the panel.
    
5. Right-click the relevant iPhone Distribution file, and click "Export "iPhone Distribution:...".
    
6. Before you can save the iOS Distribution certificate file to your computer you are prompted to enter a password for the file.
    
7. Create a password, and then re-enter the password to verify it.
    
8. Click OK.