The Triban RC500 page says you have the short cage version derailleur.
The Shimano groupset line-up page for Sora shows the short cage derailleur (RD-R3000-SS) has a max sprocket size of 32, so a change to a larger range cassette is not feasible with the current derailleur.
If you replaced the derailleur with a long cage version (RD-R3000-GS), you could go to a 34 tooth sprocket.
You might not need a longer chain, but it’s generally recommended to replace the chain with the cassette, unless the chain has very little wear.
As for as possibility of making this change as an inexperienced mechanic, all the procedures are quite straightforward, and its possible to find very good guides and videos on how to do everything. Park Tool Repair Help is a particularly good source. However, to change the cassette you need a couple of special tools - a chain whip, a cassette lock ring tool and a large wrench. To change the chain you would need a chain breaker tool, and possibly quick link pliers.
If you choose to try to re-use your chain, do not install the cassette and then just see if the chain will go onto the new 34 tooth sprocket, if it’s too short it can cause damage. Break the chain and use the chain sizing procedure to see if it is long enough. If it is re-join using a quick-link connector. Make sure you get a quick link that is for 9 speed and for whichever brand of chain you have, Shimano, SRAM and KMC etc.