You can add email addresses and change your login (default) email in your user settings here.
To do this, you'll need to first add a new email address by clicking "Add another email".
You can then provide your email address, optionally specifying the institution affiliated with that address if applicable.
After clicking the Add new email button, please check your email Inbox for the confirmation message, and follow the steps in it to confirm your address. (You may need to check in your Spam, "Updates" or "Promotions" mail folder for the email.)
If your university is part of a group premium subscription, adding the email address on your account to the email address which is part of the group subscription will upgrade your account to the premium level.
Note that you can only login to your Overleaf account with the Default email. If you would like to change your login email, make sure to click on the Make Default button next to your chosen email address after adding it in Account Settings.