Editing parts

I have a piece for piano, voices, and cello. When I go to ENGRAVE, I only see the full score. When I go to PRINT, I can select the cello part to print individually, but I cannot edit it. How can I edit just the cello part?

In this dropdown Menu, in Write or Engrave Mode, choose the part you want to edit.

Also possible: when in the full score, select something in the cello part and press W.

Thank you!