How to move images to a new folder on a D3100?

I made a new folder in my Nikon D3100, but how do I move old images i've taken into a new folder?

You don't

Those folders are used to manage your images BEFORE you copy them to your computer. Once all your images (and folders) are copied to your computer, you are supposed to format the memory card using the format feature on your camera.

Sample

When I have more than one assignment a day, I make a new folder for each assignment on the memory card(s)

Memory cards are NOT to be used as a long term storage device

The SD card is a TEMPORARY storage medium to hold the pictures until you can download them to a computer.

Move your old images to your PC and view them there.

Connect the card using a card reader to the computer and do the transferring there. You do realize you may not see the pictures ever again during Preview?

The memory card is not a permanent storage device. From there, you should copy the pictures to your computer then burn to a CD for safekeeping.