There are two options.
1. Right-click a PDF and click "Open With..." (use Shift-Right-Click if this option does not appear).
2. Browse to where you put SumatraPDF and select this program as default.
3. In Calibre, go to Preferences -> Change calibre behavior -> Behavior. Uncheck PDF under "Use internal viewer".
Calibre should now use the default PDF-viewer for the system, which you set to SumatraPDF in step 1 and 2.
Alternatively, if you want a different default PDF-viewer but still want SumatraPDF for calibre, you can install the Open With plugin:
Preferences -> Get Plugins to enhance calibre.
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