as you might have noticed, it is not possible to view and update the
view of PDF files with LyX 2.1.3 if you use Acrobat Reader DC as viewer
for PDF files.
The solution it to replace the file pdfview.exe on your PC (in the same
folder as the lyx.exe) with this one:
Viewing and also updating the view is then done via LyX's view button
(the update button won't do anything visible).
Please test and report back if it works for you.
If it doesn't work, please write what version of Acrobat you are using
and what Windows. Please also write the name of the Acrobat program
window when a PDF is opened (e.g. "Hello.pdf - Adobe Acrobat DC Pro").
many thanks and regards