You can change the labels for Likert and Slider question types in the question settings.
Changing Slider Labels
By default, slider questions do not have end-point labels. You can change the end-point labels for slider questions in question settings under "Slider".
You can also change the Lower Bound Value and Upper Bound Value. These values will appear when respondents are interacting with the slider, and are included in CSV exports.
Changing Likert Labels
Likert question labels can be customized in question settings, by checking off "Enable Custom Likert Labels". You can choose the number of scale points and specify custom labels for each point.
Custom labels will appear at the top of the Likert chart, while items will appear on the left side.