To update/modify the welcome email text:
- Go to <your moodle url>/admin/tool/customlang/index.php?lng=en (assuming that you are using the default language)
- Then click on "Open language pack for editing"
- On the following page just select "moodle.php" from the components list.
- Enter "newusernewpasswordtext" in the string identifier filed and click show strings.
- Copy and paste the text from the "standard text" to the empty text field under "Local customisation" column
- Click save after you've modified the text.