Upgrading an OpenWRT router

I have a Linksys WRT1900AC router (the version 1 hardware), currently running OpenWRT 15.05-rc3. Now that the final release has been made, I'm contemplating an upgrade. According to the OpenWRT wiki, I'm supposed to download an image with a file name ending in "sysupgrade.img" rather than "factory.img". However, I can't find such a file on their download site. The best I can find (again via the wiki) is https://downloads.openwrt.org/chaos_calmer/15.05/mvebu/generic/openwrt-15.05... Suggestions on how to perform a safe upgrade would be welcome. As with any piece of networking hardware, the desire is to minimize risk.