Rename screen Session

TL;DR

C-a :sessionname mySessionName

Details

This is,

  1. Attach to the session in question.
  2. Press Ctrl+A.
  3. Type :sessionname mySessionName – yes, the first colon is needed there, no extra spaces.
  4. Type Enter.

Renaming without attaching

Screen's -X switch lets you rename a session without attaching it.

screen -S 8890.foo -X sessionname bar