Hi Hive account@sahriarbappy, you can edit your posts using the Edit button at the bottom of the post:
This site is still in Beta, so they are still working through some bugs. Sometimes the Edit button won't work, or you won't be able to post the edit, but come back later or the next day and it should work :)
RE: Steemit You can not edit old posts Can change this so we can edit old posts? I can not see a way to contact steemit