How can I extend my retirement visa to a 1-year visa in Thailand on my own?

Express visa 0890245255

If retirement visa it's easy..not so marriage

Just Google list of documents required to apply for Non-O visa 12 month extension of stay. Write down the list of documents and forms, gather it all together and go to your local immigration office in Thailand. You'll find they're usually very friendly and helpful. If there's anything missing or something extra they want they'll let you know. Go a few days early in case you need to get something and go back. But if you have what's on the list you should be fine. Some offices want the bank statements dated the same day you apply some offices allow a 2 or 3 day window. Again Google will give you a list of what you need. Also you can go on-line to the Thai government immigration WEB site and download the forms in either pdf or Word format. Give it a go before spending money on an agent or lawyer.

its very easy ，do it by yourself

Why not go to your local immigration office to ask them what documents or paperwork is required?

Seriously? You have the visa, you have the funds. It's easy to apply for a 12 month extension of stay yourself.