Sometimes the latch is spring loaded and all you need to do is pull it out from the side of the door and rotate. However, if it is this easy, you probably would have completed it already. Usually requires pulling out the lock mechanism but it should be reasonably easy. I recommend opening it carefully and taking a picture of the mechanism before you move anything. There are usually various springs in the case and figuring out where they go, facing which direction can be challenging.

First step is removing the doorknobs and spindle. Then there may be a screw in the middle of the mechanism in addition to screws top and bottom. The middle one is an adjustment screw and isn't removed to get the lock out of the door. If the mechanism doesn't want to move, put a screwdriver through the spindle hole and wiggle it around to get the lock to move.

You remove the handle. Then you slide the latch out and spin it around 180 degrees so that the point catches the face plate. Slide it back into position (obviously the spindle needed removing before spinning it around 180 degrees). With the spindle back in position relocate the two handles and screw them onto the door. The latch will now engage as it should do.