Interleaving reversing

MATL, 8 bytes

t2L)P5M(

Try it online! Or verify all test cases.

Explanation

t     % Implicit input. Duplicate
      % STACK: 'abcdefghi', 'abcdefghi'
2L    % Push [2, 2, 1j]. This represents 2:2:end when used as an index
      % STACK: 'abcdefghi', 'abcdefghi', [2, 2, 1j]
)     % Get entries at those indices
      % STACK: 'abcdefghi', 'bdfh'
P     % Flip
      % STACK: 'abcdefghi', 'hfdb'
5M    % Push [2, 2, 1j] again
      % STACK: 'abcdefghi', 'hfdb', [2, 2, 1j]
(     % Write entries at those indices. Implicit display
      % STACK: 'ahcfedgbi'

Alice, 10 bytes

/ZY
\IOR@/

Try it online!

Half of the bytes of this program are spent on correctly formatting the source, the actual commands are just IYRZO, because Alice has just the right builtins for this task.

Explanation

As I said, the mirrors (/\), the newline and @ are there just to make the ip move in the right direction and terminate the program at the end. The actual code, linearised, is the following:

IYRZO
I      Input a line
 Y     Unzip it into its even positions and its odd ones
  R    Reverse the odd positions
   Z   Zip it back again
    O  Output

Quite straightforward, I'd say.

Jelly, 7 bytes

s2ZU2¦Z

This is a full program.

Try it online!

How it works

s2ZU2¦Z  Main link. Argument: s (string)

s2       Split s into pairs.
  Z      Zip/tranpose, grouping characters by the parity of their indices.
     ¦   Sparse application:
   U         Upend; reverse both strings in the pair.
    2        Replace the second string with the reversed string.
      Z  Zip/transpose, interleaving the two strings.