First, open your image in Photoshop.
Next, on the top tool bar, go to 'Image'. Then slide down and click 'Image Size'.
This will open the 'Image Size' dialogue window.
Here we can see the image size properties in pixels or measurement units like inches and centimetres. We can also see the Resolution of the image.
To change the size of the image, choose inches or centimetres, then input the desired width or height. When the 'Constrain Proportions' check box is ticked, entering one value will automatically adjust the other.