When you import a People CSV, Yapp works out whether each row is someone it already knows — by Unique ID if your file has one, or by comparing names and job details if it doesn't. Rows it recognizes update an existing person instead of adding a new one.
It's worth knowing what "update" means before you upload, because Yapp rewrites all of that person's details from your file. Anything your file doesn't say is treated as empty, rather than as "leave this one alone":
A column you leave out of your file is cleared.
A cell you leave blank is cleared.
A blank Photo URL removes that person's photo. This one catches people more than anything else here.
The only exception is the Unique ID itself. Leave that cell blank and the person keeps the ID they already had.
Why a partial file is risky
It's tempting to build a small file with just the columns you want to change — names and new job titles, say. Don't. Every column you left out gets emptied on everyone the import matches, so that file would wipe your whole list's emails, phone numbers, photos, and bios.
The safe way to make a change
Export your People page as a CSV.
Edit the file you just downloaded, leaving the columns you don't care about as they are.
Upload that whole file back.
The export contains every column Yapp knows about, already filled in, so nothing gets cleared by accident.
One more thing to watch
Yapp checks for duplicate Unique IDs inside the file you upload, but it doesn't compare them against the people already in your app. If you reuse an ID that already belongs to someone else, the import will overwrite that person's details rather than warn you. Starting from an export avoids this, since you can see which IDs are already in use.