I am staying in Singapore & would like to know how could I upgarde my Forms Central account from free to a paid plan. Please advise.
Basically you need to have a credit card and billing address in a country that FormsCentral is sold in. You can use your subscription from anywhere, so you can purchase if you have a card/billing in a supported country and use in Singapore.
This is the list of countries supported for purchase: http://forums.adobe.com/docs/DOC-1375
You should review this FAQ which helps users purchase when they are in another country - the country that your AdobeID is associated with needs to be the same as the credit card and billing address, this FAQ describes how to fix it when that is not the case: http://forums.adobe.com/docs/DOC-2984