The data is: 13, 17, 52, 56, 65, 35, 77, 39, 15, 37.
First, sort the data:
\[
13, 15, 17, 35, 37, 39, 52, 56, 65, 77
\]
The median \( x \) is the average of the 5th and 6th terms:
\[
x = \frac{37 + 39}{2} = 38
\]
Now, replace 35 with 53:
\[
13, 15, 17, 39, 37, 52, 53, 56, 65, 77
\]
The new median \( y \) is the average of the 5th and 6th terms:
\[
y = \frac{37 + 39}{2} = 38
\]
The difference between \( x \) and \( y \) is:
\[
\boxed{6.5}
\]