You can download and install the software update From the phone.
If you want to update your LG G4 to Android 6.0 by flashing the stock firmware, From below link you can download the stock LG G4 Android 6.0 firmware H81120i.kdz
Download /H81120i.kdz from LG Server: H81120i_00_0201.kdz
To learn more about the T-mobile LG G4 H811 Software Update instructions, please visit: https://support.t-mobile.com/docs/DOC-22421
The Update is also available for download from “LG Bridge”.
T-mobile LG G4 Software Update history:
Your software is up to date. There are no updates available.
Your phone can update to software version H81120i: