To edit an existing Html Snippet navigate to the snippet that you would like to edit using the tree menu.  Click on the item in the tree using the left hand mouse button and the snippet details will appear in the right hand frame of the browser.

A sample screen is shown below:

Edit the fields required and click Save Details.  The snippet will then be updated in the repository.

Note:  Any instances of this snippet that have already been used in Page Content will NOT be updated.  The snippet change will only apply to any new usage.