Under blender 2.79 and previous there used to be a hot key for this that has been removed from 2.8 series.
The operator is called Flip Region and it switches the side a region (like the tool shelf or the properties sidebar) is docked to.
In 2.79 you could toggle it by hitting F5 while hovering the relevant area, in 2.8# you have to call it from the menu.
The nuance is your mouse cursor must be hovering the region you want to affect when invoked, so move your mouse over to the properties region, press Space bar or F3 (depending on your key config) type
Flip Region and execute while the cursor is hovering the relevant sidebar.
For 2.9 Series.
You need to have Developer Extras on. Then you can follow the 2.8 series directions.
- Top left of Blender's window, click Edit for a dropdown menu.
- Then drop down to click Preferences. You'll get a Popup Window.
- From the Popup Window, be sure to click Interface to open up Interface preferences.
- Expand the Display panel.
- Then turn on Developer Extras under the expanded Display panel.
You can now follow the 2.8 series instructions. Go back up to the top of this answer for further instructions.
No major UI changes have happened in the search menu in 2.90