Find out the largest tooth your RD will handle and what its chainwrap is. Chainwrap is the the total difference in range for your crank and cassette. 39/50 crank = 11, 12/26 cassette = 14, so your current chainwrap is 25. A medium cage RD will normally handle up to a 28t and a long cage may take as big as a 36t cassette but also pay attention to the smallest cog it can accept. The easiest solution here is to use a mountain bike RD and cassette such as an 11/34. This gives you good gearing for downhills as well and this setup is quite common on touring bikes.

Using a triple is also a viable answer but will not give you quite as low of gear ratios with your current cassette if you really crawl up hills. Going to a 30/39/52 will always spread your gears out in both directions with more steps in between. Unless you are carrying a touring load or really like to spin up hills you may not want smaller than a 26t in the rear. This is likely more expensive and would require changing the cranks, FD, and bottom bracket.

You may answer the question for yourself by playing around with a gear calculator like the one here. The main factor here is how fast you pedal, cadence, when climbing.

Good luck.