Changing a fixed wrapper to a fluid wrapper (or the reverse) is a simple process. You can change it as many times as you want, and it doesn’t matter if you have blocks in the wrapper already.
Open Grid Mode
Open the Visual Editor and get into Grid Mode. Here, you’ll see all your blocks and wrappers for the layout you’re viewing. Make sure you’re in the right layout!
Hover over the Wrapper
When you hover over the wrapper, a control that looks like a cog appears on the right.
Right-Click the Settings Icon (Cog)
Right-click that settings cog to reveal the right-click menu for the wrapper.
Click “Change Wrapper to …”
From the right-click menu, choose the Change Wrapper to … option. This will either say “Change Wrapper to Fluid” or “Change Wrapper to Fixed.” Click the option you want.
And you’re done!