first of all you should create your save's property
for this I've put a picture of how to do that
you can choose 'name' , 'type' , 'scope' and 'value'
select user scope so the value can be changed by code
now you can change it form your code : My.Settings.MyPropertyName
you can use a butoon to save your textboxes:
here we have two properties named "UserName" and "Password"
Private Sub btnUpdate_Click(sender As Object, e As EventArgs) Handles btnUpdate.Click