There are ways to improve your chess without actually learning more about the game, simply improving brain function can make a big difference. For most people this means a cup of coffee with lots of sugar and maybe a cigarette, but there are healthier alternatives that are based on research.
First and most importantly it’s essential to avoid dehydration, and coffee tends to cause this because it encourages the body to give up water. A much better idea is to water or green tea, the latter probably requiring that you bring your own given the limited range of refreshments normally available at tournaments.
As far as food is concerned then fish and eggs are the best brain foods plus green leafy vegetables, blueberries and whole grains being very good too. There are also a number of supplements that aid cognitive function, for example Gingko Biloba is known to help with memory and concentration and has the benefits of being easy to obtain and inexpensive. The same is true of dark chocolate, the cocoa beans containing flavanols which are known for their brain enhancing effects.
Going beyond just food there’s a whole World of activities one can do, for example exercise has been shown to have beneficial effects on the brain. Here’s someone whose books I’ve read lecturing on the subject, and it’s amazing how few people have bothered to watch: