How do I file a complaint for a AT&T Wireless store?

I've been trying to change from post paid to prepaid account and I'm getting the run around now for a week. I called 611 and was told to go to AT&T store. I went to 1567 Highlands Dr NE, Unit 130, Issaquah, WA 98029 and was told the change from postpaid to prepaid is no longer done in the store,  I was told this by their manager, Quang.

Called the 611 again and was told it ONLY be done in AT&T store, so I went to 6150 E Lake Sammamish Pkwy SE, Issaquah, WA 98029. There were 3 employees there, including their store manager, and not a single other customer. After looking at my account they told me they can't help me and I need to call 611.

I called 611 and was told to go to a store. With Rene, the agent from 611 we called the store and talked to the manager there, Stephan, he told us not to go to the store because they can't help us, even though the 611 agent, Rene was claiming the only way to change from postpaid to prepaid is start the process at the store. The store manager at 6150 E Lake Sammamish Pkwy SE, Issaquah, WA 98029 just hung up on us and told us not to go there again.

I've spent a whole day going back and forth between stores and calling 611 without any resolution. I've been a customer for over 15 years and have never seen such incompetence!

I would like to know what is the formal way to report the manager at that AT&T Wireless store and also, change my plan from postpaid to prepaid!