The real way to repair an increase you missed is to rip it out and do the whole thing over. This is because the increase would have added yarn to the fabric--and more yarn would have been added to each row or round in the column above the missed increase. Any after-the-fact repair is just a re-distribution of existing yarn, and does not really serve to "increase" the width or diameter of your knitting. So that's the long of it and the short of it. Rip it out and start again.
So--how come you're still reading if you now know the right thing to do?
Perhaps it's because we're not in the ideal world just yet.
Well, if you've just got to (or if you're just going to) slam in a missed increase, here's how to do it.
1. (Above) The missed increase should have been created where the blue tail is (circled in red).
2. (Above) Grab the tail and twist it up to form a loop (this is easiest with a crochet hook). Diligently work as much extra yarn towards the loop as you can--drawing excess yarn from as far as several stitches away from the new loop, on both sides. Draw the tails of the stitches above through the newly-made loop, one at a time, creating a new column where none lay before. For each tail drawn through, again work as much excess yarn as you possibly can from the neighboring stitches in that row towards the newly-made stitch.
As you can see, this trick pretty well distorts the fabric. Nevertheless, if you find yourself compelled to use this trick, and are feeling bad about that, here is a little consolation for you: knitting is so stretchy, so accommodating, that even a messy, distorting, after-the-fact "increase" like this will even out over the years until it becomes far, far less noticeable than when it was new--especially if the increase was only pulled up over a few rows, and especially if the yarn is wool. (Don't ask me how I know...I take the 5th!)
Addendum--be sure to read the comments--there is an excellent suggestion from Mishka on how to make this little trick less obvious.
You have been reading TECHknitting on: adding an increase missed in the row below