Raymond Weil ladies watch replacing battery

[lost in espace]

I bought my wife a RW Tango as a present about 6 years ago, she has never worn it of course. But tonight she decided as she was going out to a posh do she would put it on, and surprise it has a flat battery.

I have been tasked with the job of sorting it. For the sake of an easy life I agreed. Is it necessary to send it to the authorised service centre as recommended by the RW website or can any watch repairer do it, or is it even DIY'able? I cannot see an easy way into the case.