Can I edit someone elses form

If you are a sub-user of the owner of the form and you are given edit rights, you can.

User guide: How to Share Forms with a Sub-Account User

OR

if you were instead referring to the form submission data, that you want to edit, you can do it if you have the EDIT link to the specific submission data of the form.

User guide: How to Let Users Update Their Form Submissions at a Later Date

I hope this help. Let us know if you need further assistance.