Hi,
For the new user registration you can change the subject "COM_TAXIBOOKING_EMAIL_ACCOUNT_DETAILS" using this string. You can create a language override or can change it directly from the com_taxibooking.ini.
And for the email body you have to use this string "COM_TAXIBOOKING_EMAIL_REGISTERED_BODY".
You just have to careful of '%s' while changing these strings.
For the new order reminder you can change the subject "COM_TAXIBOOKING_NEW_ORDERS_REMINDER_SUBJECT" using this string. You can create a language override or can change it directly from the plg_system_tbschedules.ini. And you cann't change the email body for new order because it is generated dynamically from tbschedules plugin.
Hope this helps.
- Shariar.