You have multiple options to change the playback volume of Instacast, depending on the hardware and software capabilities of your iOS device.
One thing you can always do is changing the playback volume directly in Instacast. If you are using iOS 4 and you have the system player not enabled, you can simply tap twice on the podcast image or on the video to get the playback options to be displayed.
Another thing you can always do is use the hardware buttons on the left side of your iPhone or iPod Touch. If you happen to have an iPod Touch 1st generation, there are unfortunately no hardware buttons for changing the playback volume.
Something that should work on every iOS device is using the volume control on the lock screen. If you carry your device around and the screen is locked, you can tap the home button to activate the screen. Then tap the home button twice to show the playback controls.
And finally if your iOS device is multitasking capable and Instacast can play in background you can tap the home button twice to bring the task switcher onto the screen. Then swipe your finger right twice to get to the volume controls in the task bar.