Install

To install this add-on, choose [edit] > [preferences...] > [Add-ons] > [Install...] > choose downloaded ZIP file.

To enable the add-on for future uses, check the box next to its name to enable it.

'Symmetrize UV Util' panel will be appeared in the UV Editor (while a UV map is displayed for editing).

Axis setup

You need not set up anything, except that the mirror axis of either your UV map or mesh is not X but Y.

Both UV and mesh are mirrored along X axis (Default)

Both UV and mesh are mirrored along Y axis

UV is mirrored along X axis, but mesh is mirrored along Y axis

Adjust axis position

You can now symmetrize UV map everywhere by putting 2D cursor as pivot point. This may be useful for UDIM texture.