If your educational establishment uses a PDF for their online-generated results, which often matches the paper version sent out or given on Results Day gatherings, then I would suggest "pranking" them (and pranking them only) by editing the file on Adobe Acrobat DC. You can download a free trial and figure it out from there.
If you're creative enough, you can replicate a paper version through manually creating it using the relevant tools. This is much harder if they use a personalized template with lots of complex shapes and unavailable fonts.
Never use faked results to an employer. Never use fake results to con your parents into thinking you've achieved - they'll eventually want to know why you aren't doing a good course or doing something with those "top" grades of yours.