I want to install Kubuntu on my laptop instead of Ubuntu, but the problem is its support ends soon and I'm new to this environment, so I don't know what to do when the support ends.
Do we get an update or do we need to clean install the newer version?
If you're talking about 14.04 the support already ended. Regardless of the flavor please install a supported release. Any LTS release is supported for 5 years (all the others have 9 months only). The newest LTS - 18.04 - has support until 2023.
And yes, any release can be upgraded on-line.