Default [Q] "Singnature verification failed" in CM 10.1 and Nook Tablet

Hi all.

I am using CM 10.1 on Nook Tablet,

Today I downloaded CM 10.2.1 (by both ways, using Wifi on Tablet and using PC then copied the zip file to tablet), but when I tried to update, the error "Signature verification failed" appeared.

Any help is welcome.