What do I enter in the outgoing mail server area when I’m trying to change the email account

Different email providers have different Mail Servers and, very often, different settings. You need to check the settings with your own email provider. Your provider is likely to have a help page that lists the explicit settings.

If you are experiencing difficulties, you might find it far simpler to add a new account from scratch - and allow the setup wizard to work out the settings for you.

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

If you are using the Mail app, you will need to ask your internet service provider - or enter it/choose it from the available list to see if it will populate automatically.