By default the email address displayed on your proposals is taken from your login address. This is done because it is possible to have multiple users within any one account. And you may want to send or create a proposal on behalf of another user.
If you are the only user in your account, then to change the email displayed on your proposal from the login address to your business email address, simply click the settings icon while in 'Edit' or 'Preview' mode'. Then select 'Do not display". This will tell Nusii to display your business details instead of your personal details.
If there is more than one user in your account and you wish to create or send a proposal on behalf of another user, then simply select the appropriate team member from the drop-down menu. Their details will then be displayed in the proposal instead of yours.