I have an iPhone 5s, midway through my plan, and the iPhone 6 and 6 plus come out. This is probably a stupid question, but can I upgrade to the iPhone 6 plus right now? And if I can, what's the price to upgrade? My plan is $70 a month.
You will need to pay off your current Tab, if you still have any. You can choose to buy the iPhone 6 on Tab S or Tab M, the latter is probably the best choice for you since you'll get the iPhone 6 for $375 and the iPhone 6+ for $500 :)
Also, if you don't have the extra money upfront to pay off the tab in store, you can access your self serve account and charge your tab balance to your next bill, then you can walk into your nearest Koodo kiosk or retailer and get the new iPhone on a new tab. I'd recommend the tab medium like Jonathan and Sophia suggested above, it's the cheapest deal over the course of two years.