Hi there! Thanks for commenting in our community! You can adjust line styles colors and endings by selecting the "Line Style" and "Endpoint" features in the top toolbar of the document. You can see from the screenshot below the options that are available. 

If you have drawn a line on the document and would like to change it you can simply select it on the page (it should turn blue to show it has been selected) and make the desired changes. 

Hope this helps! Let me know if you have any other questions.