To customize label text for a single label, start by creating a new layer. In the Data field, pick the tileset that contains the label you want to override. In the filter field, create a filter that excludes everything except the label you want to override:
Right after creating your new layer, the map should look the same as it did before, because the layer has not yet been styled. From here, with your new layer still selected, type whatever you want in the Text field field:
You can even combine tokens, which let you pull text field values out of the data, with your own text:
Combine multiple layers to have more control over exactly how labels appear:
Layers higher in your layer list will be shown with higher priority that layers lower in the list, so place your layers with customized labels higher in the list.