With Image size and resample checked a new replacement image will be generated. Image size with Resample not checked the image pixels will remain the same ones only its print resolution will be changed. If you use resample the new generated image will be of lesser quality.
If everything was that easy! You can change dimensions in inches as previously explained but things are not that easy.
Image dimension is measured in pixels or measurement unit like Inch multiplied by Resolution. If you want to work with Inch as unit then you must know print or display resolution to set proper Resolution in advance before changing dimensions in inches, mm, cm...
In short: with pixel-based images, there is no such thing as inches. There is only pixels.
Physical size is assigned with the pixels per inch number (ppi). You can assign any ppi number to any file - but in most circumstances you need a minimum pixel density, thus limiting the possible size.
The sooner you come to grips with this basic fact, the more efficiently you will work in Photoshop.