As everyone has already said, flush and shift.
Fair warning: It is not unusual for the flush-and-shift method to take much longer than you think. I had one set that I'd WD40 and shift for 5 minutes straight, and it took a few days of this until I was guaranteed not to have a missed downshift.
If it won't downshift at all, you may have to put the shifter cable in a vise so you can put sufficient pressure against the system for the mechanism to begin clicking back.
In other words, if you are not prepared for the process, you'll get frustrated. Do a little spray-and-shift every day, multiple times a day when you can. Be patient, and you'll be rewarded.
-Kurt
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Last edited by cudak888; 10-07-19 at 05:24 AM.