click on the "image" tab at the top of the screen, click on "image size" in the new box look for resolution, default is probably 72, change it there.
Be warned a 2.5mb file will jump to approx 46mb doing this.
I dont really understand why you want to do this ? i assume maybe you think it will give better printing results, it wont affect the digital resolution.
Have a look click here for an explanation and screenshot of how to do it.