Patience and more of the same, patience
A watch kettle will never boil
There are factors:
Some which are beyond your control and those with are totally under your control
1 - 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.
2 - Running behind a VPN which may increase Packet losses or corruption of the download
3 - Turn off any Third Party Two Way Firewall and try again
4 - If via wifi & too many Other devices using the wifi at the same time reducing the Bandwidth to your computer.
5 - Best Results are via Ethernet Cable to Router
6 - 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
Try Again using the Software Update Applet in System Preferences / Setting