Open the image in Paint.NET
Duplicate the layer
Add a new layer above the image
Fill the new layer with a grunge or noise texture
Set the texture layer blend mode to Overlay, Multiply, or Soft Light
Lower the texture layer opacity
Use Adjustments > Add Noise on the image layer
Use Effects > Noise > Add Noise for a rougher look
Use Effects > Distort > Frosted Glass for subtle wear
Use Effects > Distort > Dents for uneven damage
Use Effects > Blurs > Motion Blur lightly for aged softness
Use the Eraser tool with a rough brush to remove parts of the image
Use a distressed brush or stamp to chip away edges
Add scratches with the Paintbrush tool on a separate layer
Use the Magic Wand to select areas and delete small sections
Feather selections before deleting for softer worn edges
Darken edges with the Gradient tool or a vignette effect
Lower saturation with Adjustments > Hue / Saturation
Add contrast with Adjustments > Brightness / Contrast
Apply a slight sepia tone for an aged look
Save the result as a PNG or layered PDN file