It can sometimes be much more convenient to have an existing door hanging on the other side of the door frame than you currently have it. This works best on painted door and door frames as you can paint over afterwards.
There are several ways to do this. The way I like to do this is to:

Take the hinges off the door frame and the door. (it helps to hold the weight of the door if you put a wedge under the door)

Fill the old hinge rebates in door frame either by:
1 Cutting some fillets of wood the size of the old hinge rebate. Pin and glue them in place(using wood glue and nail with panel or moulding pins), When the wood is dry sand smooth then paint

2 Simply fill with good quality 2 part wood epoxy wood filler then sand smooth and paint

Spin the door horizontally through 180 degrees so that the existing door handle is on the new opening side.

You need to take out the latch and rotate that through 180 degrees. To do this you'll have to unscrew one of the handles and then remove the central spindle. Now you'll be able to unscrew the latch, rotate it and screw it back.

Put back the spindle and handle as they were.

Now you can re-hinge the door on the other side(see hanging a door)

N.B. the new hinge position on the door will be almost where the old ones were but on the other edge of the door. When you've got the new hinges on you have a small rebate to fill. Use method 1 or 2 above.