How do I ask for an extension to revise a manuscript submitted to an Elsevier journal?
Write to the editor. This can be either via the system (see if they have a "send email" function) or simply reply to the decision letter email. It's likely the editor will say yes so you don't have to wait for a response to start revising your paper.

About the "agree to revise" button, clicking it tells the editor that you will be revising your paper after all. Some authors abandon their papers after a revise decision - I don't think they abandon the paper entirely, but they stop caring about the submission to this journal. It's good practice to remove these dormant papers from the system; otherwise it'll get cluttered. If you do not agree to revise, then after a reasonable length of time (several months, say) the desk editor might remove your submission. Since it's only been a week, you don't have to worry yet. There's no need to agree to revise for now: in the unlikely event the editor does not approve an extension, it's a (minor) administrative hassle to change the article's status again.