Look at fixing the underlying problem
If the door is no longer aligned well (look at how even the gap around it is on each side), you might want to look at fixing the underlying problem. As @dandavis suggests in a comment, start by just making sure all of the hinge screws are tight. If that doesn't fix it and the door is still sagging a small amount, you may be able to square things up by sticking cardboard spacers behind hinges as shims where needed. That may realign the latch with the striker plate and you won't need to do any "surgery".
Striker Plate - two approaches
If you still need to adjust the striker plate, there are two approaches. The problem is that the existing hole no longer lines up with the latch. You can either make the hole bigger or move the striker plate so that the existing hole lines up with where the latch is now. Enlarging the striker plate hole requires removing steel from the striker plate. Moving the striker plate may require a little simple wood working. Depending on the tools you have at hand, one approach may be faster and easier than the other.
If things haven't shifted much and you have a Dremel tool (or a file and some time), Ecnerwal's answer gives you a way to figure out where you need to enlarge the hole, and you can probably do it in place. If things have shifted more than a trivial amount, moving the striker plate may be faster and easier. The method I'll describe to locate where it needs to move can also be used as an alternate way to identify where to enlarge the striker plate hole if you want to solve it that way.
Door jamb hole
With either solution, if the door has shifted more than a small amount, the striker plate may not be the only problem. You can get the latch to clear the striker plate only to find that it no longer clears the hole in the jamb behind the striker plate.
My approach starts with removing the striker plate, which you can do with either approach. While it's off, close the door and verify that the latch still clears the hole in the jamb. If the alignment is off enough that the latch isn't clearing the hole, use a Dremel tool, a drill with a rotary rasp, or a file or chisel to enlarge the jamb hole in the area needed.
Now to the striker plate, which at this point has been detached. An easy way to locate where the hole needs to be for the latch is to use the latch to position it. On the back side of the striker plate at the top and bottom, stick some double-sided tape or a thin layer of tacky putty such as is used to stick light things to a wall.
Slide the striker plate over the latch in its normal orientation and slide it against the door. Slowly close the door while holding the striker plate in place against the door so that the latch being partially pushed in by the door jamb doesn't knock off the striker plate (it might help to use a tiny dot of the tacky putty or short length of temporary tape). With the door closed, use something thin to push the striker plate into position against the jamb, and apply enough pressure so that it sticks there. Make sure the striker plate has fully separated from the door if you've used anything to hold it in place, then turn the knob and open the door.
Mark the outline of the striker plate on the jamb. If you used clear double-sided tape, you will probably be able to see part of the original screw holes through the holes in the striker plate. Otherwise, you can remove the striker plate, pull off the adhesive material and hold it in place using the marked outline to view the hole alignment. That shows you where the striker plate and its original latch hole need to be for the latch to clear it. Now you have a choice of either repositioning the striker plate there, or enlarging its hole so that the hole extends to there when the striker plate is in its original position.
Moving the striker plate
If you move the striker plate, the jamb notch-out for the striker plate may not be big enough for where it now needs to be. You can either enlarge the notch-out using the marked outline, or reassess whether enlarging the hole in the striker plate would be easier.
To move the striker plate, you will also need to move the screw holes in the jamb. Use toothpicks coated with a little glue to pack the old screw holes. When the glue dries, sheer off the toothpicks at the surface. Put the striker plate where it needs to go using the marked outline, and mark the new centers for the screw holes. Drill pilot holes and screw the striker plate in place.