My door frame is 6 1/4". How do I modify a store bought door frame to fit?

72 1/4" is pretty short as the standard is now 80". Assuming the width is OK, you would cut from the top, not the bottom. Carefully separate the head jamb from the side jambs, cut the side jambs and reinstall the head jamb. You will probably have to cut the hinge sets out or the door and also reset the strike.

Confused - you are keeping the original door, and fitting it to a new frame?
Whether that, or you bought a set up door, you buy or make extensions to add to the frame, on the outside edges.

Just saw the first response. Thought you meant the width of the jamb. Which is it.

Where are you located? Doors come in different sizes and you can't always count on cutting them down. https://www.menards.com/main/c-7566.htm
Exterior doors almost always have some trim or styling which may require cutting some from the top and the bottom to keep it looking symmetrical

A custom made door might be your best bet... as was mentioned cutting anything but a solid core door is going to be a problem. And few exterior doors are solid core (and interior solid doors are even more rare.)

Not sure about what is being addressed. Poster notes " How do I modify a store bought door frame to fit?" Door frame, not door! We keep addressing doors.