[Theme: Modern Style] How to move sidebar from right to left?

The sidebar is already floated left.

Try floating the most left element, right.
https://www.w3schools.com/cssref/pr_class_float.asp

Try using Firefox with the Firebug add-on for this kind of CSS work.
https://getfirebug.com/

And, yes, you’ll probably have to make a few other changes to move the sidebar properly.

can someone give me detailed instructions how to switch to sidebar on left side?

This is not a WordPress issue, try asking forums that specialise in CSS.
Exampled https://csscreator.com/forum .

I’ve find a solution ??

in file: screen.css ( modern-style/css/screen.css )
change line 101
form: {float:left;margin-right:10px;}
to: {float:right;margin-right:10px;}

tnx all ??