there's no direct way, but you can follow this simple steps:
- Uninstall OpenTTD;
- make a fresh instalation of OpenTTD 1.5.0 (using the
installer here
);
- Goto installation folder (usually in C:\Program Files\OpenTTD or C:\Program Files (x86)\OpenTTD, depending on your system);
- Download BTPro client ZIP file available
here
;
- Extract "openttd.exe" and "lang" folder to OpenTTD instalation folder (as told above). Overwrite them files there.
Basically it's this steps to make it work. Looks more complicated that really is ^^