How do I change the resolution in GIMP?

To change the resolution of an image in GIMP, you'll use the "Scale Image" dialog. Here's how:

Open the Image in GIMP: Launch GIMP and go to File > Open to select and open your image.
Access the Scale Image Dialog: Navigate to Image > Scale Image. This will open a dialog box where you can modify the image's resolution and dimensions.
Set the Image Resolution (X and Y Resolution): In the "Scale Image" dialog, you'll see fields for "Width," "Height," "X resolution," and "Y resolution." The "X resolution" controls the horizontal DPI (dots per inch), and the "Y resolution" controls the vertical DPI. Changing these values affects how many pixels will be printed per inch. Higher values result in more detail and a larger file size for printing, while lower values result in less detail and a smaller file size.
Input the Target Width and Height Values: Enter your desired width and height in the corresponding fields. Ensure the chain icon between the Width and Height fields is linked if you want to maintain the aspect ratio. If the chain is linked, changing one value will automatically adjust the other. If not linked, you can change them independently, potentially distorting the image. The units for width and height can be pixels, inches, millimeters, etc. Choose your desired unit from the dropdown menu.
Choose Interpolation Method (Optional, but Recommended): Below the width and height fields, you'll see an "Interpolation" dropdown menu. This determines the algorithm used to resample the image when scaling. "Lanczos (Sinc)" is often a good choice for downscaling as it generally produces sharper results. "Cubic" is also a frequently used option. Experiment with different methods to see which one works best for your image and intended use. This option can significantly affect the final image quality.
Click "Scale": Once you have set the desired width, height, resolution, and interpolation method, click the "Scale" button to apply the changes.
Export the File: After scaling the image, go to File > Export As... and choose your desired file format (e.g., JPG, PNG, TIFF). Select appropriate export settings for your chosen format (e.g., compression level for JPG) and save the file.

Example:

Let's say you have an image that's 1200x800 pixels at 72 DPI, and you want to change it to 300 DPI for printing without changing the pixel dimensions. In the "Scale Image" dialog:

Width: 1200 px
Height: 800 px
X resolution: 300 DPI
Y resolution: 300 DPI

Then click "Scale". The pixel dimensions will remain the same, but the image will now be considered to have a higher print resolution.

If you did want to change the physical print size while maintaining 300 DPI:

Set the resolution to 300 DPI for X and Y resolution.
Change the width and height to the desired physical size (e.g., in inches or centimeters).
Click "Scale." The pixel dimensions will automatically adjust based on the new physical size and resolution.