I don't think you are able to change the legend title, but you can hide it.
At the same section, Colors and Legend, there is a checkbox at the end where it says "Show legend title".
You just have to uncheck it.
This way Qlik Sense will only display the measure names.
I hope I could help!
Looks like this is a problem with the June update, but I was able to find a workaround for now:
Appearance > Colors and legend > Colors - Custom > Colors - By dimension > Change the label for your color expression to whatever you want your legend title to display.
Hopefully Qlik will fix this problem eventually.