Look at the left and right side gaps. If the left is wider than the right then you want a small shim (piece of wood, cardboard, whatever) behind the top door hinge plate, between it and the frame. You probably won't need more than about 1/32" to 1/16", something like one or two layers of cardboard from a cereal box if you lack the wood working equipment to make a wood shim that thin. If you can see it installed and that's a problem then you can paint it white before putting it in.
If the right is wider than the left, you want to chisel out a small depth deeper for the hinge plate to sit in, in the door OR frame behind the lower hinge plate. If a large difference is needed, some people might take a little material off both to make it less noticeable.
If both the L & R gaps are about the same then ideally you would do both of the above, half as much, but from the picture it looks like there is enough gap on the left that I might try just a shim on top first as that is more reversible by taking it out, than removing material from the door or frame at the bottom then trying to put it back.
Chiseling out either to a deeper depth you would put the chisel at the top and bottom of the gap at a right angle to the wood and strike it to cut to the depth you want removed, then come back and chisel at a steep angle to remove the vertical chips of wood along the grain till it's an even depth. I suppose you could do that with a big pocket knife or even a steak knife but it would take a LOT longer.
After an adjustment to the top or bottom you may want to loosen the middle hinge if present and retighten it down to adjust for the new difference between the top and bottom hinge.
If either of those methods aren't enough because the door frame has shifted too much, then you can do it the hard way and take off the frame and cut it down on the back (so it fits *any* door with standard dimensions), or the easier way is take off the door and trim it down, so it only fits right in that frame. Cutting either down and having it look good/true, you probably need a table saw or something else with a guide, rather than a hand saw.
If the house continues to shift or this is a seasonal thing with different humidity levels causing the wood to swell and shrink, it may need readjusted again or still need the door cut down.
Before you do any of this, make sure the sides of the frame are nailed in good, not just coming loose.
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