How do I reset an FPGA development board to its factory settings?
Like most FPGA development boards, you can restore the factory shipped configuration by following the same steps you use to load your own firmware, except that the configuration bitstream is ready-made and does not need to be recompiled. So instead of opening and building a project, you instead just go straight into the programming tool. (If this were Xilinx ISE this tool would be called Impact, I think on Altera Quartus it's just called Programming Tool... I don't have the latest Quartus so I could be wrong about that. I mainly use Xilinx.)

For the Terasic DE2-115 development board (Altera Cyclone IV FPGA), it looks like the board comes preloaded with DE_115.qsf. If you can't find this file on your own system, grab a copy from the ftp server:

ftp://ftp.altera.com/up/pub/Altera_Material/12.1/Boards/DE2-115/DE2_115.qsf

(I just followed the link in Peter Teoh's comment, went up to the parent directory, and found the .qsf bitstream file with the same name as the board.)

Restoring the factory shipped configuration is useful if you want to verify that the board is working correctly, if (as a student) you're getting ready to sell it on eBay for textbook money. In this case you mentioned you're returning a borrowed board, so that is good to leave it in a known-working state for the next user. Often the source code of this initial configuration may be provided as a working example. This might be called factory test or some other similar name, most likely just follows the same name as the development board.