Your MailChimp add-in for Outlook should find the latest data every time it communicates with the MailChimp website database, but if it doesn't, you can force it to re-load all the data about each list:
1. From the MailChimp addin toolbar in Outlook, select 'Configuration / other settings'.
2. Click the button at the bottom marked 'Re-load MailChimp Data'
This should refresh all the data in your database (it may take a little while if you have a big list).
Please also note if you change fields in your MailChimp list, especially if they are mandatory fields, you will need to update the field maps in the add-in to reflect the new fields.