A slightly more tactful way of putting it is that the upgrade options for your bike are fairly limited. For example, if you want to put trigger shifters on, you’re going to have to go for ones that are compatible with your six speed cassette, which will be pretty low quality, and because you’d have to buy them new, relatively expensive. The other option would be to replace the cassette, mech, chain etc too, which would obviously cost quite a lot, and leave you with a drivetrain which was limited by the rest of the bike.
My recommendation would be to leave your bike as it is, use it to increase your fitness etc, and start looking out for good quality but older used parts in the classifieds, collecting them as you can afford them and gradually putting together a proper mountain bike. For about £300 all in you will be able to build something which is massively lighter, stronger and safer, that will be able to take you up and down pretty much anything you’ve got the balls/confidence to ride.
You’ll come across a fair bit of bike snobbery and general cockish behaviour on here, but you’ll also get a surprising amount of friendly and helpful advice about how and what to put together to get the best bike for a small budget that will really put a smile on your face.