I've got a prepaid (DTAC happy simcard) with 199thb/week "unlimited" internet access. I want to use another type of internet package, so I need to cancel the current one. Their website says I shoud press *10041 and then 9 - it didn't work. They are saying something in Thai which I don't understand.

Just dial 1678 and tell them to cancel your package. I think you'll have to enter another number for English. The automated service will prompt you. I think that additional number is 7.

Gary is right - you need an operator to do it for you. The automated systems can't cope with a change like that in English

Yes I had same problem and hand to contact them direct to cancel the recurring package. I now subscribe manually when I need it. Can also be done via email in English on their web site if you have registered and you don't want to phone.

I usually call 1678 to cancel a data package, they say it may take a few hours but I usually get an SMS in a minute or two, after which you can re-subscribe to another plan. AFAIK, there is no way to do this via eServices, or the Android app./

http://www.dtac.co.th/en/prepaid/products/Happy-internet-package.html

How to cancel internet package services

• IVR automated service dial (free of charge)
  
• dtac call center, dial 1678 press 77 (3 Baht/call, include 7% VAT )
  
• dtac service centers and dtac centers nationwide