How to update all emails on I pad from I phone

Mail on your iPad cannot be “managed” from your iPhone - or vice versa.

When using IMAP, assuming that the email account has been added to each of your device(s), will independently download a copy of your email messages when fetched from your Mail providers email server.

The Mail server will keep the original email messages until such time as they are proactively deleted from either your iPad or iPhone. When an email is read, it will show as such on all devices. IMAP is intended to keep mail and associated status consistent between all devices that can access the account.

If instead you are using POP3, email clients can be configured to (a) download and leave a copy on the server - or (b) download and delete from the server. As such, email messages may be deleted from the Mail Server as soon as they are downloaded by a single device. 

If supported by your Mail provider, it is possible to configure a mixed environment where some devices are using IMAP and others POP3. Doing so requires considerable care - as misconfiguration can lead to unexpected behaviour. IMAP is the more modern Mail protocol that, for most domestic Users, is the preferred option.

Attempting to mix IMAP and POP3, between multiple users that can access the same accounts, is just asking for trouble. To avoid message loss or unexpected behaviour, you should use IMAP.

Whether this is possible, and, if so, how to do it, depends completely on your email service provider and the type of email service you are using (e.g., POP3, IMAP).