Golden Pass

I am sure you can buy a first class upgrade at the ticket office, but my question to you is: are you sure the Swiss Pass will save you money? There are other options that may be more cost effective, like the Swiss Card or the Half Fare card. The Swiss Pass is very expensive, and unless you are going to spend the entire time riding the rails, it is hard to make pay.

Thank you for the information. I am still calculating the Swiss Pass Saver.

What is the best way to purchase it?

You can purchase a fare upgrade at any train station once you are in Switzerland. The Swiss Pass has always saved me money. In addition to inter-city transport it also covers municipal transport, free entry to most museums, and steep discounts on the train/cable cars to the high-peaks.

Here is the website to purchase.

http://www.swisstravelsystem.com/en/tickets-en/swiss-pass-en.html
I'd also check the price at Rick Steves' agency, they have a better price when you do the currency conversion.

Swiss passes cover a lot (rail, bus, boat, and museums, plus lift discounts of 50% off in most areas) and give you hop-on convenience for those other trains that you don't have to reserve. See Rick's description at http://www.ricksteves.com/travel-tips/transportation/trains/switzerland-rail-passes. If you have a 2nd class pass, then the 1st-class "upgrade fee" for any particular travel day is to pay the price difference between a 1st- and 2nd-class ticket. On the price list at http://www.goldenpass.ch/CMS/default.asp?ID=256, this difference is 24 CHF. Pay it at any Swiss train station, perhaps the day before, if that's more convenient than the morning of departure.