You cannot change the Dots-Per-Inch (DPI). To get more detailed, sharper images in Photoroom, you can increase the image resolution (see Resize your designs in Photoroom). The higher the number of pixels, the better the image quality.
DPI represents how many dots of ink are printed per inch and is not directly relevant for digital media, where pixels are the standard unit of measurement.
Most Photoroom exports are saved at 72 DPI, which is a common default value in many digital image editing tools. A higher DPI would not make your image look better on screen.