Wireless Protection: 5Ghz - Howto Modify Channel width?

Question

I'm currently playing with the Wireless Functionality of integrated XG 1* Series. (I have here a XG 135w). I noticed, that when I create a SSID in 5Ghz Band, it's Channel-width is allways 40Mhz. I couldn't find any place to change this back to default 20Mhz, what would make more sense from my point of view. does anybody know how to do so? 
 In UTM, there we had the possibility to modify channel width via CLI, is there a similar option in XG Firewall?

HuberChristian · Answer

Thanks Luk. I allready did this, when I opened this case. Because I know Sophos's XG Support is currently nearly unusable, I tought, maybe the community could help faster. 
 To share my Knowledge with the community: 
 It's currently not possible to change Channel width on SFOS (XG Firewall), they asked me to open a feature request (What I will not do, because Sophos proofed in the past, they don't care on very most parts of those fature requests)