How can I change User Roles such as Company Admin and Translation Manager?

Only the Company Admin can make the changes.
It can be changed from Company Settings>Members on the Settings page. 

To assign a certain role, please click the icon next to the user name in the members list (from left: Company Admin, Translation Manager).

Click the icon again to remove the settings. If the icon is gray, the role for that member is not assigned.

* Company admins or translation manager limit is the same as the user's number limit.