What's wrong with my 640xl?

[Sumukh Phalle1992]

I have installed 10.0.10536.1004 built. I was updating it to 10549. Phone memory was nearly 2gb free before update starts. But suddenly memory goes full and update stopped. Now it's saying clear some memory before installing. What's wrong with it? Also phone becomes very slow and lagging more. Should I downgrade to wp8.1 before update? What if i don't downgrade to wp8.1? How to clear memory?

 

[Sumukh Phalle1992]

System tray shows full in storage.