Should you allow a change in password in a registration form?

I'm creating a registration/sign-up form for an app that has several pages (I have no power over changing that). The first page asks name, email, phone number and password, and upon submission of that form, the second page asks for a 6-digit SMS verification code to confirm the phone number. The rest of the pages ask for other information that's not relevant to this.

Currently, I am trying to include back buttons on as many pages as possible to give the user more flexibility/ability to change their inputs. The situation is somewhat complicated because, after the first page, their account is technically created even if the phone number code has not been entered yet. So if the user wanted to change the password but forgot the first one, they would not be able too since there's no verified contact method to confirm the person. I'm not sure what the best solution to this could be.

My current ideas are:

• Let the user change their password after confirming the one they previously entered by adding another field to that page if they go back to it. If they cannot, they would probably have to start over.
• Not allowing the user to go back to that page at all.
• Letting the user go back to the page, but the password field is replaced with a message that says something along the lines of "sorry, you are not able to change your password at this time".
• Letting the user go back to that page but only after the phone number is confirmed.