Not sure what you're asking. When will you be eligible to upgrade to a 6s? Depends on when you last upgraded and what type of upgrade you got (2 year contract, Edge). When will there be an upgrade to the next device after the 6s? Typically it will be in one year....next September and it would be theoretical the iPhone 7. The year after would theoretically be the 7s. If neither is what you meant you need to be a bit more clear.
I am Eliable to upgrade right now
And I'm looking to upgrade it online as soon as possible but it doesn't give me the option to choose the new iPhone only the old ones
You are not looking at the whole picture. The phone is half the price, but the monthly access is twice the price.
Example 1 - Contract
Phone $300, upgrade fee is $40, and monthly access is $40 per month - over 24 months
TOTAL PAID OVER 24 months $1,300
Example 2 - payment plan
Phone is $750 divided over 24 payments of 31.25 per month
No upgrade fee
access fee is only $20 per month
TOTAL PAID OVER 24 months ONLY $1,230
Don't be fooled only by the price of the phone if service costs double. Payment plan usually ends up being cheaper. Remember, you pay full price for that phone one way or another - up front payment, or hidden fees in the access charges.
Always calculate out the 24 month cost to really figure out what is the best way to go.