Installation Issues

Try logging out of Steam, or Galaxy for the GOG version. Exit the client, reboot your device and then start the client again. Log in and see if that helps fix the issue for you. 

Try verifying local files: in the Steam library, right click on the game and select Properties, switch to the Local Files tab and then click on the 'Verify Integrity of Game Files...' button. If you are using the GOG version, in the Galaxy client, select the game, then the settings icon at the top right, beside the Play button and under Manage Installation, select 'Verify / Repair'.

While in the client, within the Steam menu select Settings, then Downloads, and then try changing your Download Region.

Try exiting out of Steam and check the SteamApps folder where the game is installed, and if there is a file there called appmanifest_1086940.acf.tmp.save, delete it and restart Steam. Issues with that file can cause problems with the update or install process, such as Steam repeatedly pausing the download.

Try clearing the download cache found in your Steam Settings and then Downloads section and reboot your computer.

Try checking your drive(s) for errors. In Windows Explorer, right click each drive and select Properties, switch to the Tools tab and click Check Now in the Error-checking section. For the boot drive this will prompt to schedule a scan for the next time the system is rebooted.

Make sure you have enough free drive space to install the update/game, which generally is twice the download size, plus enough for the update/install process.

If nothing else helps, you should at least be able to uninstall the game, and try a clean install. Before deleting, right click the game in the Steam library, select Properties and switch to Local Files. Select 'Browse...', then after uninstalling the game, make sure that folder is removed or empty.

You could also try reinstalling Steam. You can effectively do this without having to redownload any games installed on the same drive by deleting everything in the Steam install folder except for Steam.exe and the SteamApps folder, then rebooting and running the Steam.exe executable.