This is childishly easy, as it can be done within the game itself. Start the Sims, choose the family that has the character in it, hold down the Ctrl key for PCs, or the Apple key for Macs, and click on the character's icon in the family gallery (the row of faces at the bottom of the screen). The game pauses, and the character's bio screen comes up, ready for editing.
The edited bio of Jeff Unpleasant.
But! The bio is altered only in the language of the installation. If my language was French, and I put up a FAM file of this character which was then imported by someone whose installation language is Danish, the bio will be something else, or empty. This is probably part of why the Sims exchange is barred to non-US family uploads. (Although a much more likely reason is strange characters that the US version can't handle, like accented e's and a's and the various Asian alphabets.)
To make sure the bio will be preserved, I open Jeff Unpleasant's character IFF - and the character IFF of every Sim in the neighbourhood - in IFF Pencil, double-click on "CTSS" (the catalogue section; even Sims have one), select first the name string, click on "Set for all languages", then do the same for the bio string. I could also uncheck "Use different languages" so that only one string will be kept - but how do I know it will be the right string?
The editing window, with the important sections outlined in red.
This will write the bio into all language versions (except the four at the bottom, probably because of the languages added in Deluxe/UL - these need the text copied and pasted into the fields).
If I don't care about the bio, I'll skip this and the bio will be wiped each time someone with a different installation language imports the character's family. The name will be wiped too, and the name from the "OBJD" section used.
Actually, this may happen even if I copy the bio to all languages in the user IFF and then save to FAM and re-import; so far I've only imported between UK English and US English, not to other languages. So to be safe, I'll do the same for the CTSS block in the FAM file before importing.
The bio is stored in the same kind of block as buyable items' catalogue
descriptions. See also Wrong names/descriptions.