I'm in a similar stage to you, so I may be able to help. I've been working away at communicating with a database for the past week, so I'm still in the early stages, but I'm able to retrieve info, update it etc, just the basics.
From what I understand, you want to send a value from C2 to your database, to update it, presumably that value has already been collected from the database earlier?
So (you probably know most of this), if you already have your value in C2 and want to send it to your database, you need to POST (not update), I think. Actually I'll be honest, I haven't even looked at the method section yet. All I do in your situation is, I send the information in the data, using the default method, then collect it in my PHP file and do the MYSql work in PHP. It's fairly simple, just takes a bit of getting used to (PHP isn't very complicated anyway, just taking me a while to get the hang of the syntax).