How to access replays in the Tenhou log:
Tenhou keeps track of your games (I think the 30 most recent) in the main menu.
On the bottom, there are three drop down menus. From left to right, they are "Menu," "Test Play," and "Cancel." Click "Menu" and pick the first option (highlighted in the pic)
Now, you should see this.
Depending on how often you play, you may either see a lot of games or very few games. There are three tabs on the top. Click on the third tab and you will see this.
Just copy and paste the URL into your browser. I think you might also be able to save them to your bookmarks so that you can watch old games again whenever you want (not 100 percent sure on this since I've never done this before) or you can send the link to friends so they can watch (also not 100 percent sure if people who don't have Tenhou accounts can access these).