Step 1: Take It Apart
The key components to look at here are the controller chip U1 and the voltage-setting resistors R1 and R2. U1 is a PJ34063 chip and the data sheet can be found online (Google it). I measured R1 and R2 at 2.47k and 8.2k respectively.
Step 2: Review the Data Sheet
Vout = 1.25 * (1+ (R2/R1))
Set Vout to 3.5V we will modify R2
After a little math and we see that
R2 = R1 (Vout - 1.25) / 1.25
And plugging in the values for Vout and R1 we get the required value for R2
R2 = 2.47k (3.5 - 1.25) / 1.25 = 4.44k
I didn't have a 4.44k resistor but I found a 4.7K so let's see if that will work
Vout = 1.25 * (1+ (4.7k / 2.47k)) = 3.6V .. which is close enough :)
Step 3: Wrap Up
Test it then button it back up and you're done.
With this technique, you can modify the output voltage of a DC-to-DC converter, but note that the voltage rating capacitor will limit the possible voltage output values.
Note 1: There are lots of different DC-to_DC converter chips (other than the PJ34063) , so you may need to look up (Google) a different data sheet, but the principle is the same.
Note 2: Some DC-to_DC converter chips are not adjustable. Phone chargers that output 5V may not be adjustable since that is a common voltage and there are dedicated chips that are set to output that voltage without using any external resistors. So try to pick a phone charger with an odd value as those are more likely to use adjustable converter chips.