As with any system software update, once installed, you cannot remove it. Restoring an earlier version of the OS would expose your device, privacy and data to emerging and known security threats and vulnerabilities.
Perhaps you refer to the update badge that indicates that an update is available? The badge that informs of an available update cannot be suppressed/hidden. The only way to remove the badge from view is to perform the update.
Consider that iOS/iPadOS updates include both functional and essential security updates that protects your device from potential compromise by malware, helps to preserve your privacy, and protect the security of your data.