You need to do the following to add new data to an existing project:

Please open the Data Center window, download newer data via the “Update from server” button.
Then open the Forex Tester 3 folder, go to the “Projects” folder and open a folder with the name of your project.

Then open the settings.dat file using Notepad and change the date in the ToDate field.
For example, set 2018.10.01 or even further date (2020.01.01). Save the file.

Then go to File -> Open Project menu, select this project and the project will be loaded and you will be able to test it till this new date.

And please create new projects to the later date (for example, till 2020 year), in this case you can always download newer data and test them in the current project.