Aruba APs are designed to use dynamic channel and power, so setting a specific channel is more difficult than choosing it. You can create your own regulatory domain profile that has only the channels you want the AP to be on, and then apply it to the AP via an AP-specific configuration:
Go to Configuration> Advanced Services> All Profiles. Expand AP. Expand Regulatory Domain. In the right pane, Type a friendly name for your new regulatory domain profile and click on Add. Click on the name of your new profile. In the Right Pane in the “Valid 802.11g Channel” section, uncheck channels 1 and 11 and click on apply. We just created a profile that will only allow your AP to be on channel 6. Next, go to Configuration> Wireless> AP configuration. Click on the AP specific Tab. Click on New. Select your access point in the dropdown and click on Add. Click on the Hyperlink for your AP. Expand AP. Click on Regulatory domain profile. In the right pane click on the dropdown for your friendly-named regulatory domain profile. Click on Apply.
Your AP should be restricted to channel 6. To adjust the channel, go back to the regulatory domain profile and check or uncheck the channels you want that AP to be on. As soon as you click on Apply it works. To revert everything, you can go back and delete the AP specific settings; the AP should resume with the ap-group settings it had before.