iOS 8 has been updated to version 8.4. The update is available via the usual method. On your iOS device, tap Settings > General > Software Update. The installation varies for each device but the download will be close to 225 MB. Use a Wi-Fi connection.
Aside from various improvements to the system, several security issues have been addressed. The issues are described in the Apple Security Page. Generally, the issues relate to addressing WiFi connectivity, WebKit malicious websites, telephony issues, Safari, Mail, Certificate Trust Policies and more.
To learn more about updating your iOS device, visit this Apple website.
As usual, it is a good idea to back up your device before installing an update.