Why I can't edit the existing chatter post?
|Knowledge Article Number||000220453|
|Description||Users may notice that they do not have option to edit specific chatter posts while they are able to edit other posts.|
|Resolution||The Edit option isn’t available on posts with link or file attachments and system-generated posts like feed tracked record updates. For community users, feed post editing is only available if the community was created using Salesforce Tabs + Visualforce or the Napili template.
Please consider promoting the Idea, Ability to edit a file or link chatter post after posting if you would like to see the ability to edit file or link posts added with a future release of Salesforce.
Edit Feed Posts and Comments