Sublime Text 3 : Can't find installed packages

Sublime Text 3 uses .sublime-package zip files to store packages. In Windows, they are stored in AppData/Roaming/Sublime Text 3/Installed Packages. The easiest way to deal with them is to install the PackageResourceViewer plugin via Package Control. Once installed, it gives you Command Palette options to view, edit, and extract packages. For example, in your case, hit CtrlShiftP to open the Command Palette, then type prv to bring up the PackageResourceViewer options. Select PackageResourceViewer: Open Resource, navigate to SASS Build, then select SASS.sublime-build from the menu. Make any changes you want and save the file - it will now be located in AppData/Roaming/Sublime Text 3/Packages/SASS Build/SASS.sublime-build, and will override the default version in the .sublime-package.

Please note that any files you extract in this way will not be updated when Package Control updates the parent package, so only extract what you actually want to edit. PackageResourceViewer has an option to extract an entire package at once, so just be aware of what will happen if you do that.

Good luck!