Go to theme settings and see if you can locate the newsletter pop-up settings. There should be a field to edit the Placeholder text.
If you have existing knowledge of HTML/CSS you can manually change placeholder text in the CSS files.
Hi, @Bigachep. Welcome to the Community! Which theme are you using?
Most wording within your theme can be changed in your language editor, by going to ‘Online Store > Actions > Edit languages’. Once inside the editor, you can simply search for ‘Newsletter’ to find the available wording that can be edited, as seen below in this example from the Debut theme:
Let me know if this helps!