LineageOs 14.1 upgrade to 15.1

[Theet]

1) Important, first you have to ensure that you have the latest version of TWRP, e.g. twrp-3.2.1-0-bacon.img, therefore you can download this file (including the twrp image that worked for me). Extract this file to your C drive.
2) Please ensure that you have USB debugging mode enabled
3) Open CMD prompt with Administrator rights and go to folder c:\adb
4) Type

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adb reboot bootloader

, the 1+1 has to reboot into the bootloader. You can also use "Volume Up + Power" button at the same time.
5) Type

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fastboot devices

6) Type

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fastboot oem unlock

Installing a custom recovery
7) step 4
8) step 5
9) Type

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fastboot flash recovery twrp-x.x.x-x-bacon.img

10) Now reboot the 1+1 with "Volume Down + Power" button and check if the instalation has succeeded
11) Click on "Wipe - choose option advanced" and delete davlik cache, cache, system
Important: download first step 12, 13 and 14 and copy them to your SDcard
12) Download e.g. lineage-15.1-20180618-nightly-bacon-signed.zip
13) Important: you have also to download e.g. open_gapps-arm-8.1-micro-20180626.zip because after installation the lineage version and before you will reboot the device you have to install the open gapps first
14) Install also Super User for lineage 15.1
15) After installing all packages now you can reboot the 1+1