Drop first segment, the entire reservation gets cancelled. Do not do this unless you want to spend your time in CA instead of Japan.

You can request to make change to the earlier flight within 24 hours of original scheduled time for the flight from SMF to SFO: https://www.united.com/ual/en/us/fly...ay-change.html

As long as the flight from SMF arrives on time, 1 hour+ is more than doable for the transfer. I had once transferred from a transcontinental flight to a transpacific flight with 10 minutes to spare at SFO due to late departure/arrival.