Once you got back all your iPhone messages and media we suggest to update to latest WhatsApp from Google Play Store!

Before updating perform a LOCAL WhatsApp backup, as shown in the interactive tutorial below. Please note: Google Drive backup will be available once you update to latest version.

Now you can safely uninstall the old version and install the new one from Google Play Store . You'll be asked to activate WhatsApp again, this is expected and not an issue (you'll not loose the imported messages).