If a parent needs to change the email they use with their Seesaw family account, you might be surprised to discover that you can't edit email addresses, this is their responsibility not yours. If they didn't sign up to Seesaw with Google, they can just update their own email preferences in their own account settings in Seesaw. BUT if they did sign in with Google, they'll need to ask Seesaw to update their email address directly, using the Seesaw support link:
https://help.seesaw.me/hc/en-us/requests/new
You should make sure that they understand that this is totally separate to CIMS, and that they should update their details there as well separately.
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You might feel tempted to resend them the family invite link or QR code; once they've connected you can delete the old email address, form the Manage Families option in Seesaw class settings. But unless they are new to the college (in which case why are they already changing their email address?) I would not advise it. If they use your invitation to sign up with a new email account, in effect be a completely new Seesaw account, and so will not be connected to any other Seesaw content from previous years.