You lift up on the release handle. If the mechanism is working properly the step should drop down and lock into place.
There are notches in the track that hold the step in place once extended. These notches wear over time and need to be reshaped to hold the extended step properly.
You should inspect the latch mechanism that holds the step in the travel mode. Look for a broken spring or rusty components that prevent the mechanism from securely locking the steps in the travel position. It may only require a good cleaning and lubrication.
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