nighthawk r8000, what would happen if i download latest firmware

> If i download latest firmware, what would happen? [...]

   I have no idea.  What's the current firmware?

> [...] Can't access router since I typed wrong ip (192.168.1.99), won't
> let me do anything...should I factory reset?

   "Can't" or "won't let me" is not a useful problem description.  It
does not say what you did.  It does not say what happened when you did
it.  As usual, showing actual actions (commands) with their actual
results (error messages, LED indicators, ...) can be more helpful than
vague descriptions or interpretations.

   Where, exactly, have you "I typed wrong ip [address] (192.168.1.99)"?

   How, exactly, does it _stop_ you from doing "anything"?

> [...] should I factory reset?

   I don't see why not, but I have no idea what your problem might be.