I don't think there is any way to update the Ajax Explorer since it is in the firmware part and we do not have access to the firmware. Maybe a future firmware upgrade will contain a newer version of Ajax Explorer.
You can check a wish list (things that we asked DLink to implement into the new firmwares) by checking the sticky topic at the top of this forum (or by
clicking THIS link )
Regarding the 1Gb limitation, well, i never saw any limit on the filesize that i can transfer to the NAS.
You should write what OS you're using and how are you trying to upload the file (FTP, through the network, using Ajax Explorer, ...), maybe then somebody will be able to help you.