How do i adjust the duration of an echo?

You can only adjust the parameters of a plugin after you've used it, in Multitrack view - just open up the effect and edit it there. If you apply an effect in Waveform view, that's a permanent print of it once you've saved it - there's no way to alter that at all. If you want more detailed information about what you need to alter, then you will have to tell us very specifically which effect it is, and what the settings are. Unless you did this in Waveform view - in which case you'll have to start over.

I should perhaps add that if you want to do this in Waveform view, then make a copy of your original file and work on that. The benefits of this approach should be obvious!