You can download the update directly on your device through the Windows Phone Store or by using the link which follows:
Press the Search button on your Lumia phone, tap Vision and then scan the following barcode by pointing your device camera toward it
WeChat for Windows Phone version 4.0 has the following new features and improvements
- Free HD video calls.
- Support for video messages: record a video and share it with your friends or loved ones.
- Play audio messages in Handset Mode to keep your messages private.
- Free cross-platform individual and group messaging with text, voice notes, images, videos, location data, and more.
- Secure, locally stored message histories always accessible offline.
- NO ADS ANYWHERE!
- Private, group-based photo-sharing social network.
- Incredibly easy friend adding by shaking your phones together, scanning QR codes, syncing with your phone contacts, sharing IDs, or using location-based services.
- Blazingly fast registration with your phone number.