Sorry if this has been asked before, but I couldn't find the answer.
I am trying to just change one think on 28 SMs connected to one AP. I just want to go from 10mhz channel width to 15mhz.
Do I need to create a template, then upload that template to all SMs? How can I make sure I only change the channel width and not anything else?
The below guide from the Help page outlines the steps needed to create and apply a configuration template. The general process is to apply the changes you want to a test device, download its configuration file and then remove all parameters except the one you want to apply to devices while maintaining the structure. The guide provides in depth details.
I do recommend validating your configuration file with https://jsonlint.com/ after you finish your edits to check if the basic structure is still valid.
One other thing to consider for your use case is to allow both 10 MHz and 15 MHz at least initially so that after applying the update the SMs can connect back to cnMaestro and report success. Otherwise the configuration attempt may timeout and be marked as failed. After all devices, including the AP, are running on 15 MHz you can apply a second template telling the SMs to only use 15 MHz if you wish.
Please note that if you use a template to change the bandwidth of the AP too, you may need to create a separate template since it might use a different parameter for bandwidth.