Reinstall OpenGAPPS Open OS 16.10.0

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You first need to download the latest OpenGAPPS version & have it on your SD card or internal storage (if not encrypted), so you can pull it form there when in TWRP.
See also:

Update Guide for a modified FP Open OS (with Gapps, Xposed and more)

When having other packages installed with FP Open OS (e.g. opengapps or xposed) it is possible that you will get stuck in a bootloop after an update, if not installing these packages again. Because these additional tools are installed on the /system partition, you need to reinstall them after every update, since the /system partition gets overwritten during the update. However, you should not loose data or settings, which are stored on your /data partition. If you have not modified your FPOOS …