Before you proceed, you need Spaces installed on your phone.
How to edit a post/notice/content posted?
Step 1 - Open the Space > Navigate to the Post/Notice for which you want to change the content (You will be able to edit the content for the notice that you have posted only.)
Step 2 - Click on the three-bullet icon at the right corner of the post.
Step 3 - Select Edit from the drop-down list.
Step 4 - Go ahead and edit the content of the post and click on the Save button at the bottom right corner and Confirm (Once you update the post, the members can only see the updates post. You won't be able to revert back to your previous changes.)
You are done!