Overhead is a term used to describe only attacks with special properties. Chun’s stand :h: is a high attack in that it doesn’t hit low, but it is not an ‘overhead’. Also, stand :h: is low blocked, so they can’t duck under it and avoid block stun or anything.
Overheads are attacks that must be blocked with a stand block. For example, a jump attack cannot be crouch blocked; it’s an overhead, even though it traditionally has little mix-up potential with a 3-7 frame grounded low.
Chun Li has a few quick overhead methods in this game. Her f:h: little flip thingy hits overhead, so you can mix something like c:l:, c:m:, f:h: in with a c:l:, c:m:, c:h: pressure series, and they may get caught out by the f:h: when blocking low.
If you have a low assist, you can do something like c:l:, X-23 assist, c:m:, f:h: for an unblockable.
Just to cover some important little details about Chun’s normal/pressure game:
Her c:h: sweep does not have the great reach that you might think. Chun’s furthest reaching basic normal is by far her stand :h:. Second to that is her c:m: low poke. c:h: comes in behind those.
Additionally, her stand :h: usually causes way less push-back on block than her sweep when cancelled into other stuff. This is what makes her :l:, 
, :h:, c:h: 4 hit series work. The sweep cannot be chained to other moves. So :l:, , :h:, f:h: is a decent way to her f:h: has use as an overhead/cross-up/continue pressure tactic, where-as c:l:, c:m:, c:h:, f:h: is really not as good since the sweep pushes them out when blocked and them stepping back just a moment (to block the f:h:) can cause it to whiff.
Another overhead technique for Chun is to jump and airdash as low to the ground as you can and then jumping . Chun Li can jump cancel either stand :h: or c:h:, so you can use it as a way to get straight back in after a blocked c:l:, c:m:, c:h: series, or as a way to get in from poking range with her stand :h:.
Stand :h: results in less pushback when you instant air dash from it than c:h:, so you get different spacings and results from what you do. Chun’s :d: air stomp is better to use than jumping from airdash when you are thinking about cross-up situations or air dashing behind the opponent, since the hitbox is directly below Chun Li and a little behind, where-as jumping is mostly in front and is more sensitive in how it is spaced for cross-ups.
Finally, Chun’s classic instant overhead trick is her :d: air stomp. You can instant overhead with it by pressing :uf: for a forward jump and hitting :d: the moment her feet leave the floor.
Because of Chun’s jump arch and the prop-up you get from connecting stomps in this game, this leaves her in useless positioning to do anything else. But that’s where the creativity of the game comes in.
For example; if you call in something like Akuma’s tatsu assist just before you do it, it will combo from the overhead and juggle the opponent forward, which gives Chun Li time to land, step forward a little and launch them with :s: off the tatsu juggle for an air combo.
Also remember to make some use of her throw during offense, especially against opponents who try to pushblock everything. You can even call in an assist to eat up the pushblock while you walk forward and throw them.
You can’t throw out of her dash, but you can dash-up throw by dashing and hitting :d: to cancel the dash just before you throw.
