I found updates to both, however they updates are dated Sept 20th and the packages file is from earlier in the month. So, when you try to run "opkg install upm" wget gives an error because the Packages file refers to and older version that isn't in the repo any more.
So, that's the issue...
When you run "opkg update" it grabs a list of the available packages from repositories that your repositories list. The issue is that list, the "Packages" file is old and refers to and old version. So, if you then try running opkg to install mraa it references an old version and the wget that tries to download that package fails.
The temporary work around is to manually wget the packages you are interested in and then install them with "opkg install "
That shouldn't be the case - I always update package index at the same time as I update packages themselves. Maybe you've just got into a time window when I was updating several packages or experimenting (which happens sometimes, but the timelines are really short)?
Anyway, please run opkg update right now and you should be good to go, I'm pretty sure package index is up to date.