All of the visuals are reflecting the data. That means they labels will be pulling directly from the data table and there is no way to change them directly. You can however, do calculations in Insights so you could do a simple string calculation to reformat the labels so they make more sense.
Another effective approach I use a lot is using a look up table (LUT). So I create a new table e.g. in excel with the one field with original field name, e.g. (OLD NAME) "population_2500" and a second field (POPULATION) with my new name "pop < 2500" and do a join (with create relationships) on the original data to the new table on OLD NAME. In the chart, I use the POPULATION field to create the chart.
