Xiaomi has not yet released Android 9 for Redmi Note 5 Pro officially but there are developers on XDA forum who have ported the new Android operating system to this device. You can download and update your device to Android Pie as of now but remember that being an unofficial status, you might not get weekly updates and there may be bugs present in this ROM.
Cypher OS 7.0.0 is based on the Android Open Source Project with a pure experience of stock Android and the developers try to maintain the features of it for its users. The main motto is to provide a rich experience and the best performance but keeping the simplicity in mind.
Custom ROM – Unofficial Cypher OS For Redmi Note 5 Pro
Open Gapps – Select Android 9.0 Pico/Nano/Micro (Recommended)
Official thread – XDA-Forum
Get your builds and changelog from the official website