Before Win 10 you can just use Settings/Options menu at any time.
In Win 10 it's more complicated. You can try the following methods.
  1. Right-click SumatraPDF.exe in c:\Program Files\SumatraPDF (or c:\Program Files (x86)\SumatraPDF) and select Run as Administrator.
  2. Go to Settings > Options and click the button to set Sumatra as the default PDF viewer.
  3. Close the program.
If issue is still not solved:
  1. In file explorer, right-click any PDF > Open with > Choose another app.
  2. Check the Always use this app to open .pdf files checkbox.
  3. Scroll down and click on the More apps ↓ link, then select SumatraPDF from the list and click on the OK button.
  4. If SumatraPDF is not in the list, scroll down and click on the Look for another app on this PC link.
  5. In the Open with… dialog navigate to c:\Program Files\SumatraPDF (or c:\Program Files (x86)\SumatraPDF), select SumatraPDF.exe and click on the Open button.
  6. The PDF should open with Sumatra, and all PDFs when subsequently double-clicked should open with Sumatra too.
If issue is still not solved:
  1. Open Settings app.
  2. Go to System > Default apps.
  3. Scroll down and click on the Choose default apps by file type link.
  4. Set SumatraPDF a the default app for the .pdf file type.

suggest change to this page