1. Open the Google Play Store app
2. Tap the three horizontal lines in the upper left corner of the search bar to open the navigation drawer
3. Tap Subscriptions
4. Tap your Squid subscription (if you don't see it, make sure you have the correct Google account selected in Google Play)
   
5. You should now see an option to cancel

If you don't see your subscription listed, make sure you have the correct Google account selected in Google Play (the one you used to subscribe to Squid Premium). You can change your Google account in Google Play at the top of the navigation drawer.

For more information about what happens after you cancel Squid Premium, see What happens if I cancel Squid Premium?