The Union button unites coaxial entities.
Quick Access Panel: 
Ribbon: Editor -> Instruments -> Union
Menu: Tools -> Modify -> Union
Select coaxial entities holding the Shift button and press the Union button.
Example: coaxial entities
before: 
after: 
Go to CADEditorX
URL of this topic:
https://cn.cadsofttools.com/help/cadeditorx_ui_eng/index.html?tools_menu_union.htm