I'm working with STM32L476 Discovery Board. But while creating a new project in my existing Keil version v4.74, i can't find the target device STM32L476. Is this the problem with my Keil version.Do I need to update the version??
The L4 series is not explicitly supported in that version. You'd probably want to migrate to uv5 to get support for newer parts.
The uv4 compiler/assembler can work with a generic Cortex-M4 part setting, and appropriate defines. Where you will likely have more headaches would be the flash downloading, where a part/family specific algorithm would be needed. ST-LINK Utilities would be able to burn a .HEX onto the part. The debugger should be serviceable, just tell it not to update the part before debugging.