There are 3 main options:
What navigation software can be used on my iPhone in Thailand?
Please note it is a paid app from iTunes. Price: USD 29.99.
If you already have purchased this app, to updated it click:
File size: 495 MB.
This is CN Thailand NT version 2015.40, while the latest Garmin City Navigator Thailand map is still 2015.20 (garmin.co.th/mapupdate).