As Jose answered, screen -d -r should do the trick. This is a combination of two commands, as taken from the man page.

screen -d detaches the already-running screen session, and screen -r reattaches the existing session. By running screen -d -r, you force screen to detach it and then resume the session.

If you use the capital -D -RR, I quote the man page because it's too good to pass up.

Attach here and now. Whatever that means, just do it.

Note: It is always a good idea to check the status of your sessions by means of "screen -list".

try with screen -d -r or screen -D -RR

Short answer

1. Reattach without ejecting others: screen -x
2. Get list of displays: ^A *, select the one to disconnect, press d

Explained answer

Background: When I was looking for the solution with same problem description, I have always landed on this answer. I would like to provide more sensible solution. (For example: the other attached screen has a different size and a I cannot force resize it in my terminal.)

Note: PREFIX is usually ^A = ctrl+a

Note: the display may also be called:

• "user front-end" (in at command manual in screen)
• "client" (tmux vocabulary where this functionality is detach-client)
• "terminal" (as we call the window in our user interface) /depending on

1. Reattach a session: screen -x

-x attach to a not detached screen session without detaching it

2. List displays of this session: PREFIX *

It is the default key binding for: PREFIX :displays. Performing it within the screen, identify the other display we want to disconnect (e.g. smaller size). (Your current display is displayed in brighter color/bold when not selected).

term-type   size         user interface           window       Perms---------- ------- ---------- ----------------- ----------     ----- screen     240x60         you@/dev/pts/2      nb  0(zsh)        rwx screen      78x40         you@/dev/pts/0      nb  0(zsh)        rwx

Using arrows ↑ ↓, select the targeted display, press dIf nothing happens, you tried to detach your own display and screen will not detach it. If it was another one, within a second or two, the entry will disappear.

Press ENTER to quit the listing.

Optionally: in order to make the content fit your screen, reflow: PREFIX F (uppercase F)

Excerpt from man page of screen:

displays

Shows a tabular listing of all currently connected user front-ends (displays). This is most useful for multiuser sessions. The following keys can be used in displays list:

• mouseclick Move to the selected line. Available when "mousetrack" is set to on.
• space Refresh the list
• d Detach that display
• D Power detach that display
• C-g, enter, or escape Exit the list