I want to reinstall Ubuntu 20.4

While Iw as updating from Ubuntu 19 to 20, the updater crashed in between, and system became unresponsive. So, I restarted the system, and then Ubuntu 20.4 loaded.

But now, it’s lagging a lot and I feel that interruption during installation is the reason, because “cleanup” process didn’t happen. How do I reinstall Ubuntu 20.4 again, without losing my data?

Always make a backup before upgrade/installation/other.

Proceed with attempting an upgrade again or re-installing Ubuntu and restoring files from a backup.