The build 291.6 is available in the Product Owners Lounge forum if you use the stand-alone version, or directly via Steam if you use the Steam version.
This version is a quick patch that mainly addresses an important issue introduced in the build 291 for the iOS runtime, background collisions had stopped working on layers > 1. PS: we'll release soon a beta version of the build 292 to address the issues fixed by Fernando after the release of the build 291.5 in the iOS/Android runtimes and/or objects, in the meantime if you got updates for the iOS/Android runtimes in the bugbox after the build 291.5 was released you probably need to reinstall them after installing the build 291.6.
In case of problem you can reinstall any other version (no need to uninstall first, just reinstall the patch of the build you want to reinstall).
If any problem with this version, please post it in the bug box, don't post it in this forum otherwise the developers won't see it, thanks!
PS: the CF 2.5 patch updates all the installed exporters, you do not need to reinstall them.