I just don't understand what to do without extension manager It would make my work very easy if it works.
You can search for and install/uninstall extensions using File -> Extension Manager (or click the "plugin block" icon on the toolbar). You can even temporarily disable extensions and re-enable them later from the Extension Manager.
I have already tried doing this:
sudo apt-get remove brackets
but I always get the following error
Reading package lists... Done
Building dependency tree
Reading state information... Done
No apt package "brackets", but there is a snap with that name.
Try "snap install brackets"
E: Unable to locate package brackets