Uncheck, "Resample Image" then change the Resolution setting to 300 pixels/(measurement). If the resolution was less the 300 to begin with, this will force Document Size in width and height to decrease. If the decrease isn't enough, turn back on "Resample Image" and set the width and height to the desired amount. Note: once you enable "Resample Image" the operation will 'lossy' meaning you will either add or lose pixels.
Others may be able to offer other descriptions/explanations. This is a common question users have, and one that takes users some time to understand completely.