Just had to go through this process so I'd like to share this for others or future me!

From Umbraco v6.2.6 to umbraco v7.5.7

From Newsletter Studio v1(something don't know) to v2.1.7

First of all install NewsletterStudio Package into Umbraco

If the tree doesn't render try to go through the steps in this thread https://our.umbraco.org/projects/backoffice-extensions/newsletter-studio/comments/75511-tree-is-not-rendering

Get all templates from ns_Template table and create an HTML file for each of them

Add each of these templates to the App_Plugins/NewsletterStudio/Skins folder

Run the following SQL:

BEGIN TRAN 

ALTER TABLE ns_EmailTrackingItems
ALTER COLUMN SubscriberId int NOT NULL;

ALTER TABLE ns_EmailTrackingItems
ADD [CustomData] nvarchar(max) NULL;

ALTER TABLE ns_Newsletters
ADD SkinAlias nvarchar(255) NULL;

ALTER TABLE ns_Newsletters
ADD [CreatedByUserId] [int] NOT NULL default(0);

ALTER TABLE ns_Newsletters
ADD [ErrorMessage] [nvarchar](max) NULL;

ALTER TABLE ns_Newsletters
ADD [Initialized] [bit] NOT NULL default('False');

ALTER TABLE ns_Newsletters
DROP COLUMN [Template_Id];

DROP TABLE ns_Templates;


--COMMIT TRAN --remove this to commit your changes (there's no going back from this)

Update NewsletterStudio Settings for tinyMCEDataTypeId attribute to a correct one:

• go to the developer section and find the RichTextEditor NewsletterStudio DataType
• Make a minor change and Save it
• get the id from the URL and update the NewsletterStudio Settings

These steps seemed to do the trick for me (so far)