There are three mods I know of that include this feature, along with a lot of other features. The Indie Stone story progression mod here (testing thread in modding forum); Awesomemod at MATY; and Klanly's debug enabler at Sims3Tools.
Depending on how far along your sim is in his/her life, you could re-create the sim by going to the mirror, selecting "change appearance", going to the "..." next to the sim icon at the bottom toolbar, saving the sim to the bin, going back to creating a new family, pulling that sim from the bin, making the desired alterations, and merging them with the existing household, moving out the "old version" of your sim, and deleting him/her.
If you want to avoid all that trouble, though, yeah, stick with the mods suggested by Srikandi.
liquidon Snake, thank you for that, but as you said it is a lot of trouble and I am not sure if they will lose all skills and rewards.
I will take a look at the mods Srikandi mentioned. Thank you Srikandi.
The quickest way to edit an in-game Sim which includes all facial editing is to go Ctrl Shift C. Type in 'editsim Joe Soap' (or whatever the person's name is and without the apostrophes) and press enter. Voila, you are back in CAS and can tweak the sim to your heart's content. When you are finished you can press the tick box and voila you are back in the game again with the freshly tweaked sim.
As a warning, using AwesomeMod, you cannot edit a sim to "your heart's content". Editsim is an unsupported feature of that mod, and the only changes you can/should make are to a sim's clothing, facial structure and hair. Changing a sim's weight and age cause problems.
I find the easiest way is to use "NRaas_MasterController.package"
It allows you to click on ANY Sims and send them to CAS to change whatever you can think of ...
Its NOT a CORE Mod and is compatible with Indy Stone and AwesomeMod
You can find it at http://ts3.tscexchange.com