Hi Alex - I assume you mean Qlik Sense Cloud Business - if so if you are the workspace owner you should be able to rename the stream in the settings are:
I show this here: https://youtu.be/MOkE1CsZzdA?t=8m28s
You can click the ... button next to the stream to rename it.
To change the name of the client you may need to contact the cloud team: using the feedback link in the menu.
Let me know how else we can help.
Also just in case you have not seen them, these may be helpful:
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update: the stream name was changed through renaming the account.
You can do it by contacting the support through "feedback" as suggested by Michael Tarallo above.
This seems like to be the only option for Cloud+ users.