Careful in (too) warm washing the knitting may suffer (structure of the wool). And if it is pure synthetic, that will not do much if anything.
If it is wool it will shrink in hot water (actually just putting wool in water as hot as your hand can bear to work in will not shrink it if you also rinse it in hot water and don't agitate it at all) but the proportions will not be what they are now because it will shrink more one way than the other (I never remember which way it works). I don't know about the "structure of the wool" suffering, but could be.
I made a hat once that was too big and it was mostly acrylic with like 10 or 20% wool so I thought maybe I could get it to shrink 10 or 20%. :lol: So I put my mostly acrylic hat in very hot water and the result was that the hat got bigger. (I also threw it in my hot dryer, but I do that with everything :shrug: )Well, maybe it wasn't actually bigger, but it got all limp and droopy. I unknowingly did something that we call "killing" to the acrylic. If you get acrylic real hot (I think even water may do it, but I haven't done any scientific tests) it will go limp. So watch out for that as well.
I had a student knitter who had a hat turn out too big and with no one around to tell her what to do she did the seam and cut thing (try seaming with a sewing machine) and I think she was happy enough with the result for a first time project. She didn't want to do it over. When you are a new knitter starting over seems daunting but us old hands start over all the time, :lol: especially with something with so few stitches as a hat. Do what ever makes you happy at this stage of the game.