1- Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.
2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading. It will only load the Minimum amount of Core Apple Processes to allow the the computer to function at a reduced Level of Performance
3 - Does the issue present in this mode ?
4 - Try again from Update Applet in System Preferences.
5 - Software on the computer interfering with the download process including AntiVirus Software
Notation: Disabling the AV Software may or may not solve the issue. Removal of the AV Software may increase the chances of the download to perform as expected.
6 - Running behind a VPN which may increase Packet losses or corruption of the download
7 - Best Results are via Ethernet Cable to Router
8 - Worst case - Restart in Recovery Mode and choose Reinstall macOS. This in theory will pull the latest version of and commence the installation.