Well I did it on my own, but it wasn't easy so I thought I'd impart some of my experience, particularly as it relates to this bike - which we love!
- Check to see where the Flat is
We were a bit surprised at the flat tire in the first place. Couldn't find anything in the tire itself so as a first step I put air back in the tube (maybe someone let the air out while it was parked in downtown Seattle?). Couldn't see any place in the tire itself but air was coming straight out of the tube right around where the valve is. The valve was in straight and appeared to be fine.
- Remove the back wheel
Easier said than done. I put the bike upside down and removed the through-axle. This was not easy at all, as it was in VERY tight. The only way I figured it out was to try it out on my other bike (we have two, one each) and it was much easier. So yes, turn the bolt on the derailleur side, counter-clockwise. First, shift into the highest (smallest) gear. Then as you pull out the axle, hold the derailleur out so that the wheel pops out. Be very aware of how the wheel comes out because you'll want to remember how it "looks" (fits) when you put it back in, in step 7below! This is probably the biggest deal of the whole operation.
- Remove the tire from the wheel
Follow normal instructions that you can find anywhere, to remove the wheel: Let any remaining air out of the tire, remove the bolt from the value so it can slip out when ready, then use the pry tools to get the rim of the tire off the wheel.
- Check the tube for a hole, rip, or puncture.
I couldn't find one at all, but I kept the tube and will check it out later.
- Replace the tube with a new one.
This was relatively easy. Put 'just enough' air in the tube so that it will hold its shape. Standard stuff.
- Fit the tube back into the tire.
This was the most difficult part for me because the tire itself - the one that the bike comes with - has an extraordinarily tight fit and does not really stretch much. I finally was able to solve the issue of 'whack-a-mole' by holding the wheel against my stomach and using the palms of both hands to force the last bit of the tire over the edge of the wheel. In addition to my two hands, I also had to use one of the pry tools.
- Put the tire back onto the bike.
Technically this was the most difficult part of the whole process. I could not find any instructions online or in any YouTube video that showed a) how to do it with a through-axel that does not quick-release, and b) how to do it on a bike that is upside down on the floor. Also because I have an SL, it's really difficult to fit the wheel back onto the tire due to the close fit of the fender. The assumption in all of these videos seems to be that you have a fancy bike stand for managing all your repairs I finally got the wheel back on by holding it very close to the axel entry with one hand, and pulling the chain and the derailer over the bottom of the sprocket, with the other ('bottom' in the case means the top, but since it's upside down, it's the bottom). The chain barely slipped onto the edge of the wheel and I had to manually pull it up onto the lowest gear sprocket. Lots of grease on the hands!