Support for Windows Phone 8.1 ended on July 11, 2017.
After December 16, 2019, the Store in Windows Phone 8.1 will no longer be available for downloading new apps or reinstalling previously downloaded apps. Users who reset or reimage their phones will not be able to recover previously downloaded apps after this date.
Included in this users will not be able to add a MS account following a reset ..
If you have an eligible phone you can update tp WP 10.
Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB,
638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, MCJ Madosma Q501.
Update using the OTC updater.