To update I uninstall the old version 2.6?
or I can start installing the new version without uninstalling the old version
tenks
Installing the new one will automatically remove the old one, unless you rename it’s directory first. So if you have anything in the Unity folder you want to keep, like custom packages etc. Move them somewhere else where they’re safe. I think a lot of people generally rename the previous versions directory, install the new one and then can run either one, just in case of bugs or other problems they might encounter.
Also backup your project directories, once you upgrade and convert them to the new version, you can’t go back (well you can, but it’ll risk breaking a lot of things).
I generally backup the packages onto a USB stick, and then install the latest version of Unity over the top of the previous version.
Then I just copy the packages back onto my PC without any problems 
I updated but now will not open unity crashes