Go to Settings > Security (or Apps ) and toggle on "Unknown Sources" to allow manual APK installations.
If you are using an older device stuck on Android 4.4.4, here is everything you need to know about the current state of the app and your available options. Current Support Status for Android 4.4.4 whatsapp apk for android 444
Go to Settings > Chats > Chat Backup and save your data to Google Drive or local storage. Go to Settings > Security (or Apps )