stuck with download complete and not updating mac os 10.15.6

Forget about updating via System Preferences> Software Update,

instead download the update from here,

https://support.apple.com/kb/DL2048?viewlocale=en_US&locale=en_US

Once downloaded double-click to install.

Try the combo update as Eau Rouge suggested. (FWIW, I always use the combo to update macOS)

If it still fails:

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

In addition to Eau Rouge's great suggestion: how much free space do you have on your system drive?