In order to change time display setting while chatting from default 12 hours clock, to 24 hours clock follow below-mentioned steps.
Login to CometChat admin panel with your access details
From left side menu click on "Settings then select web" in that section search for "If set to yes, time will be shown in 24-hour clock format"
Change your Setting and click on "Update Settings" Button.
Clear Cache from left side menu
Clear browser cache