When you have an image large in dimension size but low in resolution, you can increase the image’s resolution without sacrificing quality in Adobe Photoshop CS6 using Image Size dialog box, follow below steps:
Step 1: Open an image in Adobe Photoshop CS6.
Step 2: Go to Image > Image Size. Image Size dialog box will show.
Step 3: Uncheck Resample Image checkbox so that Photoshop will not add any pixels.
Step 4: Enter a resolution in the Resolution text field. Photoshop keeps the same pixel size but reduce the document size when you enter a higher resolution.
Step 5: Click OK.