You can edit a pre-header by going into the source code (html coding) of your email template.
To do this click on the Source Code icon and then do a Ctrl and F command on your keyboard (this does a word search) and type the word ‘preheader’ as shown below:
This will then show you the preheader text. Just replace the text with something else to change the preheader on your email template as shown in the example below:
Please remember to save any changes by clicking on the blue OK button at the bottom of the Source Code window.
Please also send a test of your email to be sure the change has been made.