upgrading an image

Is this to satisfy external requirements? This isn't as simple as it sounds. You may not be happy with the result. That has nothing to do with Photoshop, but the very nature of digital images.

First of all, a digital image file is pixels, not kilobytes or megabytes. File size on disk depends on compression or not, and how much and what type of compression. Pixels are finite and discrete, and the only way to increase pixels is to resample, i.e. invent new pixels from the existing.

This sounds like a very moderate size increase, so it should be possible without too much degradation. See Derek's post for how.

If this is a jpeg, you can just resave at a higher "quality" setting (less compression). This will bring the file size up without touching the image resolution. But note that jpeg compression is destructive, non-reversible and cumulative, and there will always be slight degradation. For a single resave it might not be noticeable. But it builds up.

You can use the Image > Image Size dialogue feature to resize an image. You can experiment with the various settings in the drop-down menu for enlargement. As it works by resampling adjacent pixels the success of the technique with depend on the quality of the original image. The process will tend to soften images so a little use subsequently of the Unsharp Mask may help