Only group administrators have the right to edit or delete posts of others.
To do so, select the specific group and search for the post that should be edited or deleted. In the upper right corner there is a three dots menu to edit or delete the post. Choose one of the two options.
When editing the post, you will have to save your changes again. Once the post is modified, it will be marked with "Edited".
To delete a post, select "Delete" in the three dots menu.