Many Approaches …
There are a number of ways to encourage users to enter optional data that are each valid and usable in their own different contexts. Here are some prototypical examples:
Edit Profile/Edit Account
A basic utility function. StackExchange places this in a utility navigation on the profile page. You might also expect to find it in a global utility nav. It's standard language and unambiguous … but I see it less and less, and you can probably do better.
Edit Your Information
A little more plain-language than "edit profile." It communicates the same concept but uses friendlier language. 37 Signals uses this on their profile page.
Facebook users this on their timeline page (what used to be the profile page).
This goes nicely with a profile completion meter to encourage users to fill in the remaining optional information. LinkedIn currently uses this in the right sidebar of the profile page.
Add [a thing] To Your Profile
Use a little alert box or widget to encourage users to add optional information piece by piece. LinkedIn currently uses this beneath your profile summary in the main column.
"Improve" Your Profile
This is language I've only seen used on LinkedIn. It uses a more positive connotation than "edit."