It sounds like the cable has gone slack, that's all. They stretch from new over the first few hours riding, and simply need adjusting at the barrel adjuster where the cable outers come out of the shifters.
Put the bike in a stand, or turn it upside down, then shift the gears up to the smallest cog on the cassette, and middle chainring. Test the tension of a length of the exposed inner cable (between cable-outers). Turn the barrel adjuster anticlockwise and you'll notice that the exposed cable will tighten up. When you have taken most of the slack out, try shifting the gears onto the next cog while turning the pedals. If it shifts cleanly without hanging up or skipping, try the next gear and so on. If it doesn't shift cleanly, then try another turn or so of the adjuster. Once you are happy with it shifting from small to large cleanly and with only one click of the indexer, then try coming back down the cogs. The chain should drop cleanly down,. If it doesn't, then the cable is now a bit too tight and wants backing off little by little until it does. Most ofit is trial and error I'm afraid!
The chain jamming whilst changing up to the larger chainring is probably also due to slack cable. Again, just take enough slack out of the cable using the barrel adjuster so that the cage can shift the chain as far as it needs to .
Have fun!