To upgrade the standard non sat nav unit to a sat nav system is not possible.
If your talking about the OEM honda system then the entire dash board Loom and harness will need to be replaced. I have read that one guy tried to attempt this but gave up due to the vast amount of work required.
If you want additional audo options, Xcarlink systems are the best way. They plug into the back of your standard OEM unit (cd changer port) and will act as an aux option on the front of your stereo. they also offer ipod connection, streaming from your phone via bluetooth and hansfree
http://www.xcarlink.co.uk/product.php?productid=302&cat=3&page=1
If i had the non sat nav version Accord i would get this fascia which converts the lower cubby section to a double din. This will let you buy any sat nav type double din and fit it in this area. There are various looms and harnesses available which will enable you to fit the double din headunit within your standard OEM headunit and will work together via Aux i think.
http://www.ebay.co.uk/itm/Two-Din-special-Car-Radio-fascia-for-HONDA-Accord-2002-2007-Black-/281807785159?hash=item419d0d8cc7:g:G9QAAOSw9r1WAlTw
https://www.google.co.uk/imgres?imgurl=http://i590.photobucket.com/albums/ss347/mtsumustang/Accord/IMG_1244.jpg&imgrefurl=http://www.driveaccord.net/forums/7-audio-electronics/9955-double-din-7th-gen-4.html&h=1024&w=683&tbnid=fsTSWsSne6USnM:&docid=29ui-6AGeuSiwM&ei=evW8VtCqMMLePPT4nKAD&tbm=isch&ved=0ahUKEwjQz-PFzPDKAhVCLw8KHXQ8BzQQMwgkKAcwBw
There are chinese units avilable which replaces your entire OEM headunit with a sat nav type but i have heard these are not very good. Has slow interface and generally pretty erratic.
Although worth a try if you can afford it
http://www.ebay.co.uk/itm/In-Dash-DVD-IPOD-RDS-GPS-Radio-8-Navigation-For-03-07-Honda-Accord-Camera-UK-J-/401043071114?hash=item5d6007a08a:g:HTIAAOSwKtlWo2bd
Hope this helps