I'm new to all of this too, but I'll do my best. I'll begin from the beginning so as not to leave any stage out - don't be offended!
There may be a better way of doing this, but:
plug juno g into pc via usb
Mode/USB/ select Midi
Fire up the librarian on your pc - program files/Juno g editor/librarian
In the Main window that appears, hit Read/All button
That loads all the current user patches from the synth
You now need to open a group of patches, so double click the .jgl file wherever that is.
You will now see a new window (e.g. Planet X) listing all of the available patches.
If you haven't done this already, you can preview each patch.
To overwrite a patch on the Juno G, you simply copy and paste it. so:
Select one of the new patches
Edit/Copy
Go back to the Main window listing current patches on the synth (through the window menu)
Select a patch to overwrite
Edit Replace
You will now see the patch replaced in the Main window - but you still have to upload to the synth. so,
Hit the Write All button if you've made a number of edits, or just highlight the changes and hit Write selected. Patches should now be copied to the synth for you to enjoy.
I've written this as I have done it so this should work.
If anyone has an easier way (any stages I can leave out) let us know.
As we are both finding our way with this maybe we should stay in touch and help each other out?
Barry