If there is a legit update, it should be listed in your tablet's "Settings" so try going in there to see if it is actually listed and if so try downloading it from there. NOTE: Be sure there is enough space available on your tablet before you try to do the update. If there isn't, it will always fail.
Now, if there is nothing listed in your "Settings" saying you have an update, then this is likely not a real update and you need to figure out what is on your tablet that is telling you there is an update. Bad app, virus, malware, etc., and remove the cause.