Can I go from a 235 tire size to a 245?

Usually you can go 1 size wider or narrower but not anymore without causing any problems. But if these are winter tires I recommend going narrower as they will track better in the snow.

My chart shows that the original tires on your car were 225/60R18. Check the drivers door jamb for the original spec. In most conventional cars, I always choose a tire size larger than the original, it brings the speedometer, which in most cars is set slower than actual speed, more in line with actual speed. So the 235 might be the ideal size for your car.