I have divided stereo in mono tracks. In some moments, I want to replace the part of first track (same positions) from second track…
however, it seems hard for me to do that - i have to copy that selection, then make a new selection on first track (now i’m writing down the second:miliseconds of start/end position on a paper, then selecting same in 1st track, then deleting that, and placing the copied part)…
is there simpler way?
Here is a link to a draft page in the Manual which we’ve never yet published: https://alphamanual.audacityteam.org/man/Tutorial_-_Copy_and_paste_a_section_of_audio
The editor notes (which you won’t see) say: “This is orphaned and not as good as it could be …”
Any feedback you have on trying to use that page would be useful to us 
WC
Leave them as a stereo-pair and use this plugin tool … Missing features - Audacity Support
How to install plugin … Missing features - Audacity Support
Thanks!
p.s.
How I can make a selection that is made across all tracks? at this moment, when i make a selection, it is being selected only on current track… (unlike the screenshot you made)…
You can either:
Click and drag, starting from the top track, down to the bottom track. If there are a lot of tracks you may want to “Collapse All Tracks” (Ctrl + Shift + C) first: View Menu: Track Size - Audacity Manual
Click and drag, starting from the bottom track, up to the top track.
Make the selection in one track, then extend the selection “In All Tracks” (Ctrl + Shift + K) Select Menu: Tracks - Audacity Manual
On my screenshot they are a stereo-pair, not split into two mono tracks like your example.
The repair-channel plugin tool only works on stereo tracks.