I upgraded my current version of Raspbian Wheezy to Raspbian Jessie. If you want to upgrade your Raspberry Pi too than follow these steps:

/etc/apt/sources.list
...
deb http://mirrordirector.raspbian.org/raspbian/ jessie main contrib non-free rpi
...

After that upgrade and restart your device:

>> apt-get update && sudo apt-get dist-upgrade
>> reboot