, I use one of two methods, depending on my mood:
1) Quote my own post (using the "quote" feature at the bottom of each post) and edit/update it, or
2) Copy the bit I want to update, paste it into a new Reply and update it.
The advantage of Method 1 is I don't have to re-enter all the formatting. The disadvantage is I have to get rid of all the parts I don't want to repeat. That includes the [ quote] [ /quote] tags plus any additional, unnecessary chitchat. Sometimes, particularly with my posts, that's a lot of editing (you may not have noticed, but some of my posts are... long
The advantage of Method 2 is it's more straight-forward. Copy and Paste. The disadvantage is that I have to reformat everything.
Hope that helps.