Mixing characters in a string
CJam, 11 bytes
rr{irie\r}h

How it works
This is a slightly different approach, in which I simply run a do-while loop till I have pairs of numbers left in the input.
r                 e# Read the first string
 r                e# Read the first number of the first number pair in the input
  {      }h       e# Do a do-while loop
   i              e# Convert the first number from the pair to integer
    ri            e# Read the second number from the pair and convert to intger
      e\          e# String X Y e\ works by swapping the Xth index with the Yth index in the
                  e# String
        r         e# This is our exit condition of the do-while loop. If we still have
                  e# a number on the input left, that means there are more pairs to swap.
                  e# Otherwise, we exit the loop and the result is printed automatically

Try it online here