I've got to say I got a kick out of the term "upgradation".
There are a few components to remote upgrade:
1 - Uploading your new image to the FPGA
2 - Programming the new image into your configuration flash
3 - Remotely triggering the reconfiguration.
Now you indicate that for# 1 you want to use ethernet. To me this means a NIOS processor running a TCP/IP stack. Then you can use FTP or TFTP or your own mechansim for getting the image down to the FPGA. However, if you're not using a NIOS, you've got to write some custom HDL to handle the ethernet.
For# 2, you have a few options. If you are using a NIOS, it's pretty easy to interface to the flash and program the new data in. If you are not using a NIOS, you can use the PFL or SFL (parallel or serial flash loader) megafunctions to program the flash.
# 3, use the remote_update core as std_logic_vector indicated.
Jake