For licensed users upgrades are free for up to six months from purchase date. Re-registration won't be needed. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
Your purchased installer should be here: "C:\Program Files (x86)\NCH Software\VideoPad" or similar path without the (x86). If not, use Dangerfreak's link, above. Ignore any virus prompts.
Normally, before installing a newer version, if it's been more than six-months since purchase there will be a warning prompt that your registration will no longer be valid.
There's no mandate to upgrade. Stick with what works for you. It's wise to export with a version released close in time to the one that created the project. Any unlicensed version can be test-driven, to see if there are new features, or quirks. Some features may be disabled.