Step 1: Cut Surface
Cut through the surface, making a cross shape on the grass. Using a spade laid parallel to the soil, slide it under one corner of turf.
Step 2: Lift Sod
Repeat, lifting the four corners of the turf and gently folding them back onto the soil surface, taking care not to break any of them as you do so.
Step 3: Add Soil
Add some soil to the hollow, using your hand to fill the dip, and level off the surface of the soil. Add more topsoil as necessary until it is level.
Step 4: Lay Turf
Replace the turf, and firm back into place with the back of the rake. Brush some more topsoil in between the cracks to help the turf knit together.
Fall is a good time to carry out maintenance on your lawn to get it in shape to cope with winter.