How to change the display order of proportional circles in QGIS 2.10?

In Layer Properties > Style, there should be an Advanced button which has the option Symbol Levels. Here you can define which symbol should have higher priority when displayed.

Note that I used QGIS 2.8.2-Wien to find this option so there should be something very similar for QGIS 2.10.

are you tried for invert the symbol? so lowest will be on top and highest will be on bottom