If you are still on the Android Marshmellow Version A2017G V1.0 dont forget to update at first to the Version A2017G V1.0 B11.
After that you can download the Nougat 7.1.1 (A2017G V1.2.0 b05) Zip file from the link below. I can always recommend you to download the file with a PC
Now if your download is finished you must unzip this Zip Folder 1x time and you will se a manual and and a file with the name update.zip
The last step is to put the file into the root directory of your sd card and install it over the settings.