6HP 2010 shift linkage adjustment

I posted this on Sailboat owners and was advised to post it here.
"I need help fast (before the next weekend). I had an impeller changed by a "mercury authorized service" in February. I took the boat out for the first time today. I had a problem putting it into reverse, but managed to leave the dock. Coming back in, I realized I had no reverse. Luckily, I did not use Captain Ron's docking technique - I came in slow and cautious, so I managed to ALMOST not hit the dock. Anyway, testing the motor revealed that where I used to have reverse is now neutral (but not always), and where I had neutral is now forward. There is no reverse. So, here are my question:

1. what could have happened?
2. could changing impeller effect the linkeage?
3. what are my chances of having the "authorized service" fix for free what they likely screwed up during impeller repair?
4. what are my chances of fixing it before next weekend?

I appreciate any answers.
By the way, the engine is 5 years old and purchased from West Marine brand new. Never had ANY problems before this."
I was told that all I need to do is adjust linkage, but I could not find any help on line on how to do it. Any instructions. advice would be helpful. Also, I tried to search this forum, but no luck.
Thanks in advance.

This is a 4 stroke motor?

My manuals are a little older than 2010 but assuming not much changed I don't see "what linkage" could be adjusted on the shift shaft that would have thrown it out of whack. It kinda only goes together one way and there is nothing to adjust.

And if you had the motor serviced I would take it back to them so they can sort it out - it should have been working properly before they gave it back.

Thanks. They did. My wife said it only took 5 minutes...

Sorry, as an update, they had to drop the lower unit because the internal linkage slipped and the reverse wasn't catching. all is good now!