Click on Install
Click on change update stream and check that the setting is for your installed version and not something like Plus beta.
Put in your username and password as necessary.
Select your install. Make sure you choose the correct install, if you have more than one.
If all goes well, you'll be prompted with an update.
Once you install, you need to start Trainz in order for the DBR to run.
After the DBR, follow the instructions in the announcement to apply the patch.