With the upcoming release of 1.16 on October 24th, Steam players who are on the default branch (aka what you're using if you haven't chosen 1.15.x in the Betas) as well as itch.io players who are on the Stable channel will be automatically upgraded from 1.14.17 to 1.16.0. While 1.16 will have numerous improvements over 1.14, there are also some things that are incompatible between these two versions as well as add-ons that may exist for 1.14 but not for 1.16. As such, for Windows and macOS players who wish to remain on 1.14.17 for the time being, standalone downloads are available on SourceForge. For players who wish to continue using 1.14.17 without needing to download the standalone version from SourceForge, or if you're a Steam Linux player, see below for instructions on how to do this.
Steam
Go into your Steam library and right click Battle for Wesnoth.
Click the "Properties..." option.
Select the "BETAS" section.
In the dropdown under "Select the beta you would like to opt into:" select the option "1.14.x - 1.14 old stable".