not sure how to do it.

Use the in-app proxy to trigger the normal download, and let the phone download itself.
Flow
Press the proxy in app or something, it will install a VPN on your phone.
Just go and search for update as per normal from settings, will show you the available update, download and pause it once it's downloading.
Pull down your top bar, and disable your VPN.
Then resume download as per normal.
The VPN is to allow your phone to find the new firmware, but needs to be disabled prior to installation.
Sent from HUAWEI Mate 10 Pro using GAGT