My advice is to start by switching to wholewheat pasta rather than white, brown rice rather than white etc - it's not necessarily that those things are UPF, more that you'll be starting off on the right foot with more fibre and wholegrain.
Then I'd focus on small changes, one at a time. For instance, stop buying jarred tomato sauce and start making a big batch with veg in it that you blend to smooth, reduce, and freeze in portions (e.g. Souper Cubes/massive silicone ice cube trays). When frozen, put them all in a ziploc and defrost as needed, thinning with water. These are really handy for quick pasta sauces, curries, stews or even soup (if you thin it enough!)
Then your next change could be something like stocking up the freezer with homemade sausages rolls for packed lunches - use all-butter readymade pastry and buy plain sausagemeat from the butcher. Add lots of grated veg and your own herbs and spices, bake and freeze.
After that you could resolve to e.g. only eat homemade soup, or make your own granola or similar. Then you're making smallish changes, in stages, and you aren't over facing yourself by swearing off all UPFs immediately (which is pretty undoable if you're not used to makin a heck of a lot of your own food!)
I'd also recommend following someone like Sophie (surname escapes me!) on Insta - an Irish lady who recommends lots of helpful supermarket swaps to foods with less ingredients. Hopefully someone else will remember her surname!