Hello! Glad you like them. Since you have Blender ready, the easiest way is to use Pose Mode. Select the skeleton, go into Pose Mode, and make sure to enable the X-Axis Mirror icon (top right) so that lifting one arm automatically moves the other. Once you've rotated the arms into a T-Pose, simply go to Pose > Apply > Apply Selected as Rest Pose. This sets the T-Pose as the new default for the model without you having to manually edit the mesh!