Try this :
If you do not get prompted to update try these steps:
1: Press "WINDOWS_KEY + R"
2: Search Regedit
3: Copy this "Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Rockstar Games\Grand Theft Auto V"
4: Change GTA InstallFolder path to gta path for fivem.
5: Run your GTA (not fivem) and it should update.
I havn't tried this as i have not needed to.
Or try this method which gives you the latest Launcher/SocialClub http://patches.rockstargames.com/prod/gtav/Launcher_EFIGS/GTA_V_Launcher_1_0_877_1.exe