Ok, well technically of course it is possible to upgrade individual components. Kit is built to a price and you can buy "better" bits and put them in. It's likely to be the passive components, resistors and capacitors, rather than active components, transistors and chips, that are easy to change. Or you might want to put in a better power supply.
Is it worth it? Well it comes with a lot of hassle. If you are technically competent you could do it yourself but if not you will need an engineer and that will cost. And of course you will void any warranty. Will you hear a difference? Possibly if there is some less than adequate component in the original then changing it might have an effect.
There are people who swear by the upgrading process. It's not my thing and I would save my pennies until I could afford something better.
Chris
PS It's a bit like the lads who buy a hot hatch and then change the exhaust, lower the suspension, etc. However a Hinda Civic remains a Honda Civic whatever you do to it.
PPS One pulled up behind me at the lights the other week. It apparently didn't have a driver but then | spotted the top of a head and I realised he had lowered his seat so much that he could only just see over the dashboard.