New update is taking long

Which update is it? 10.15.4?

If it is try booingt into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and see if the updating continues or not. If it doesn't continue download and apply the macOS Catalina 10.15.4 Combo Update.  That should stop the problem you're having. Generally speaking it's quicker and easier to download the combo updaters from the Apple Download site than going thru the System/Updates preference pane.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

Kill the download by clicking on the "x" at the right end of the progress bar, then try again. If it won't restart the download, then open the Terminal in the Utilities folder and paste this command line at the Terminal's prompt:

     sudo softwareupdate --reset-ignored

Press RETURN and enter your Admin password when prompted. Press RETURN again.

Try the download again using Software Update preferences.