When using File —> Print in Abaqus you can only specify DPI for PS and EPS file format. In case of TIFF it is only possible to change image size in pixels.
Graphics editors such as GIMP (free) allow you to change PPI (pixels per inch) of an image. In GIMP you can do it using Image —> Print Sise. DPI refers to physical printers so it can’t be changed per se this way.