Firmware xr1000 V.1

You don't need to incrementally do the updates, you can just upgrade directly to .86. The router should allow you to check for a newer firmware version and allow you to upgrade without doing this but it's possible they haven't updated it yet. When you download the zip file and open it, extract/click and drag the .chk file out to your Desktop for example, then if possible connect via ethernet to the router - the getting booted off would like you being connected to WiFi and it disconnecting, being physically connected you can monitor the progress more easily. If WiFi is the only possibility try to reconnect to it as soon as you notice it disconnects. Once you've attempted the upgrade with the .chk file if it still shows as .74 after its possible the browser has cached the old version so press Ctrl + F5 on your keyboard to do a hard page refresh and see if it updates then. If not, reboot the router from the interface, then access as soon as you can and try to upgrade, if after following everything it's still not working then factory reset then try the upgrade.