Some 1-to-many associations are sorted, e.g. the "My 'header file's are:" in the universe are sorted by name.
They cannot be reordered.

Many 1-to-many associations are not sorted, e.g. the "My 'member's are:" in a class.
They can be reordered.

The target semels in a 1-to-many association of multiplicity N are ordered by index, from 0 to N-1.

Select the target semel that you want to reorder.
Suppose it has index X.

Click "Move to index".

A dialog box will ask you to "Input an int:".
Enter an index between 0 and N.

If you enter an index lower than X, then the target semel moves "up", i.e. to the lower index.

If you enter an index equal to X, then nothing happens.

If you enter an index higher than X, then the target semel moves "down", i.e. to the higher index,

If you enter N, then the target semel moves "down" to become the "last" target, at the highest index.