To update/modify the welcome email text:

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