It looks like those sleepers will need to be replaced. It appears that they might have been added as a secondary aesthetic measure to cover an existing retaining wall. Is that the case? If there is another retaining wall behind the rotted timber, you could remove them. We carry hardwood sleepers if you want to remove and replace those rotted ones individually. A secondary retaining wall appears to be holding back soil, but it's hard to determine from your images.
Given this is a retaining wall, if you are unsure how to proceed, I'd encourage you to enlist the services of a landscaper for this project.