Your first version is correct. You can't use your second version because "just" (when used as an adverb of time) means "very recently", and not "now" (and the present continuous in your example doesn't go well together with "very recently"). A native speaker would certainly intepret the just in your sentence as simply/only.
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AnonymousI am already uploading a video.This is grammatically correct. The question is whether it says what you want it to say. already suggests "unexpectedly early". It would be appropriate in the following situation: