If u have downloaded the update via the updater, go to twrp, intstall the rom which will be in the downloaded_rom folder.Try uncheck md5 verification in twrp. Or u can try going under updater n select update package under options. Hope this helps. Btw, stable 18 is out but is not stable on certain device like my sg version. Better to make backup first if u are updating ur rom. I installed stable 18 n faced problems thus reverted back to stable 17.