How can I insert bytes in the hex editor?

[psifertex]

See subject.

[psifertex]

First, Binary Ninja write-locks files by default to prevent any modifications. You can disable this by clicking the lock icon in the bottom right:

And you can also disable the ui.fileContentsLock setting if you prefer this to default to off (if you use Binary NInja frequently as a hex editor or to patch files this may be desired):

Finally, just type in the hex view and bytes should be changed. If you wish to insert instead of just modify, use the insert key on your keyboard or if you're on MacOS, you may need to use an alternate hotkey: fn + i