A scammer has been trying to get people to download a fake WhatsApp update (this means it’s a trojan) that promises to provide you with a pink theme. It pretends to be an ‘official’ update, but it isn’t. The scam app is called WhatsApp Pink, and it is being spread around in WhatsApp groups. The counterfeit app propagates itself via your contacts (and is therefore a virus) and can give hackers access to your phone.
Official WhatsApp updates are distributed via the Google Play store listing. If you want an update, go look for WhatsApp in the Google Play Store and the one you have installed will have an ‘Open‘ button beside it, instead of install. If there is an official update, you’ll see an ‘Update‘ button beside it. WhatsApp wouldn’t provide an update via a group or APK link. Please also be wary of the malware and counterfeit apps in Google Play and other app stores.
APKs are not inherently bad, they are just a way to install apps outside the Google Play app store (much like a .exe or .msi file in Windows). It’s important to remember that the vast majority of app vendors don’t provide you with access to their APK files (sadly). They send them to Google Play, and then Google Play distributes them in such a way that makes it harder to obtain the APK. This means that you shouldn’t trust APKs provided by anyone other than the legitimate app vendor (which is WhatsApp).