Hey there kailashchhetri,
Thanks for reaching out to Apple Support Communities. It sounds like you’re running into issues when attempting to update macOS. We understand the importance of being able to keep your Mac up to date and we’ll be happy to help with this.
How are you attempting to update? Review the following link to make sure you’re going through the expected steps:
How to update the software on your Mac
If that’s not working, try manually downloading and installing the updating from here:
Download macOS Catalina 10.15.5 Update
In that case, you’ll need to manually start the update once the download is complete.
If you still run into issues, boot into safe mode or a new user account and try updating there:
How to use safe mode on your Mac
How to test an issue in another user account on your Mac
I hope this helps.
Take care.