>>afavou+G1
If you know where to find the mesh view of your zigbee network, the one that has all of the links between the nodes rendered using bezier curves, at the bottom of that page there are three icons. The first icon is called Network, and if you view that page there should be an option for editing the channel That the zigbee network is on.
Tapping that option should bring up a dialogue that lets you migrate to a different channel with a warning about how some devices may not automatically migrate and will need to be removed and re-added if so.