Ko Chang has many internet cafés offering web access. Charges are around a baht a minute, and the better services can be found on White Sands Beach. The outer islands also have internet access via satellite or mobile phone. Throughout Thailand local ISPs are required to connect via the Thailand Internet Exchange (THIX) at speeds that, by international standards, are slow.
There is a Thai Military Bank and an ATM on White Sands Beach, who will change US dollars travellers' cheques and offer the best rates. Hotels and guesthouses also change cash and travellers' cheques, and many resorts and restaurants accept credit cards. If you're planning a relatively lengthy stay on the island, you may consider changing your cash in Bangkok.
There is a post office on White Sands Beach, about 5 kilometres south of the Centrepoint ferry.