The RISCOS file extension is associated with Acorn RISC OS. Probably a system file used by some version of the Operating system.
How to open:
Unless there are suggested applications listed below the extension description, that are able to open the system files, there is usually no way to directly open or access system files as their functionality is only as part of the operating system. Any attempts to edit system files is strongly discouraged, as it can cause the system to be corrupted and not work properly.
How to convert:
Unless there are suggested applications listed below the extension description, that are able to convert the system files, there is usually no way to convert system files into any other file format. Any attempts to rename system files is strongly discouraged, as it can cause the system to be corrupted and not work properly.