I write click on the object to select it.
Then i wright click and select 'settings'
Go into settings and have a play.
OR
If you just want to change the colour and not twig other properies, you can click on the triangle to select it and
a box will appear in the top write corner where you can change the colour.
It took me a long time to be comfortable using Geogebra. Even now I occasionally discover new features.
I think it is a lot of fun to use.