When setting up the SSL cert, I provided an email address for lost key recovery and notices, but now I'd like to change that email.
It depends on which ACME client you're using.
With Certbot, it's "
certbot register --update-registration -m firstname.lastname@example.org".
You have to do it with each of your accounts. For example, if you have multiple droplets, or multiple ACME clients installed, and each one asked you for your email address, each one is probably a separate account.
(Having numerous accounts is fine.)
Can I update multiple SSL certs at once?
I'm not sure what you mean. The email address is associated with each account, not each certificate.