Exactly, you will find the option to turn off the WPML Translation Editor only on the original content.
When switching the editor "OFF" a dialog will pop up that allows you to determine if you wish to deactivate the WPML Translation Editor for this content only, all contents of the same type, or all site contents.
If you only need this page to be translated manually, please only choose the first option.
After that, you can edit all translations of this content manually.
Take note, if the content has translation in various languages, then you will need from there on translate all those translations manually.
Also, take note, if later on, you enable the WPML Translation Editor again for this content, then the manual translation will get overwritten and lost - which is expected, as both translation methods are not compatible.