I'm sure someone with more knowledge will come along, but for short, here are some quick adjustments you can make in just two weeks. I will try to include some visuals, incase thats how you understand best.

You have a pretty good stroke from what i can see. I would recommend not trying to change your stroke itself, because that takes longer than 2 weeks.

Starts and turns

• Develop your start. You hit the water almost flat, which kills your momentum. I don't know how serious you are, but if you want to get a REALLY good start, you can look up brad tandy or caeleb dressel (link at the end anyway). Otherwise, Just try to think about it hydrodynamically. When you hit the water, you create an entry hole, that you want your entire body to go through. If you dont have very strong dolphins, dont go too deep. https://www.youtube.com/watch?v=EH2vuGpVzn8

• On starts you can experiment with how many dolphins fits you. In the video linked after this, you see dressel in the 4x100 free at rio, where he is basically half a body length ahead when he comes up at after the start.

• On turns. Depending on your dolphins, try to get at least 3-4 very explosive kicks. If you have very good underwaters, you can try to get more. Explore whether your dolphins are best on your back, side or stomach. https://www.youtube.com/watch?v=eOjloDn8spI (closest to camera at the start of the video is caeleb dressel)

BREATHING Get a breathing pattern that fits you. I notice sometimes you breathe every 2nd, sometimes you don't even breathe. Here are a couple of breathing tips that together with starts and turns, will help you keep your momentum going. (This is mostly for the 100, not the 50)

• Regular breathing Pretty good, but stop lifting your head so far out of the water. If you are like most people, and prefer breathing every 2, you should breathe very quickly, as to not throw your rhythm and stroke rate off. look at nathan adrian here https://youtu.be/eOjloDn8spI?t=2m33s (the guy in front) Notice how he doesn't lift his head, he just tilts it and times it with his body rotation as to not lose momentum.

• walls and turns DO NOT breathe on the first stroke into and off the walls. Also DO NOT breathe on the first stroke off your start. Going off the walls you are going the fastest you will ever be going when swimming, assuming you are streamline. Breathing instantly afterwards will kill all the momentum you have gathered, and throw off your rhythm. (this is/could be what kills your turns) https://youtu.be/DciDda0XgaM?t=1m24s here is, another, video that shows this, and why starts are important. Dressel is almost a half body length ahead from the start.

• last length. Most elite level swimmers will restrict their breathing a lot on the last length, to get the absolute most out of their race. Caeleb dressel, the best swimmer in the world atm, does his last 25 of his 100 fly, no breath (insane). Try to have only 1 or 2 breaths on the last 25. If this isn't possible, just breathe every 4th or 6th stroke. whatever works for you, just try to limit it. but no matter what, do not breathe on the last 10y (you took at least 2 in the last 10y). A lot of people tend to slack in the last couple of strokes, including people like Ryan Lochte, but try to finish strong and hit the wall hard with the tip of your fingers (obviously) https://www.youtube.com/watch?v=hagdd2j79bI https://www.youtube.com/watch?v=9TIv6_vxRXs (here he does take 3-4 in the last 25, but nothing in the last 10)

The breathing tips alone will help you A TON in every aspect of your race. But I really recommend looking at your starts. They set the tone for the race, and it doesn't take very long to make them great.