Try going to options> manage licenses from the designer. In the newer versions there is an extra step to click the gear cog at the top right of the interface. Once you get there you can edit the email address by clicking the 'edit' button next to machine serial number.
I am using Alteryx latest 2019.2.0 Trail version. However alteryx was previously downloaded by other person and has completed his trial version. I am not able to change emailID its uneditable. Please refer screenshot.
Click on Options> Manage License. Then the window would be displayed. On top right click on Setting icon. Then Out of 2 displayed values, Select the first then you see option to edit you email address.
But Problem comes here is that, License number is blank for me. Any help.
If you are at 'Activate Designer Activation' and do not have access to click 'Options', click 'Activate by License Server'. You will see a settings icon at the upper right side of the 'Manage Licenses' window. Click that icon and select 'View Partner Data License Manager'. You will see and option to 'Edit' your 'User Serial Number', which is your email. Enter new email, then click OK. Close the window. At 'Manage Licenses', click the settings icon again and select 'Change License System'. Click 'Continue', then 'Activate New License'. Now it will show the new email.