Usually cats confine their clambering nature to the couch , some climbs on the curtains (LOL) but it happens that their skills sometimes take them up a tree. Unfortunately, the way up for cats is much easier then the way down.
There is no advice on how to get cats back to ground. You’ll have to be very patient as the kitty probably scarred and any attempt to reach her may send her higher in the tree. let her relax and calm down and maybe she’ll climb down by her own. Talk to her in a calm voice, call her name and try making noise with her treats bag, toy she love or any other noise she usually responds to and comes running to you. You can even put her beloved food under the tree for her to smell and come down.
Try to find what was the reason for her to climb the tree. Maybe it was a dog or other threat, if you can remove the guilty from the cat eye sight.
Do not frighten her as it can cause her to jump and injure herself.
If after few hours the kitty is still in the tree try the local shelter for name of a trained animal handler that will do the job for fee.
If you want to do it by yourself, be sure to wear gloves to protect you from her claws. You know how she use them when frightened. Also, you’ll need to grab her at he nap of the neck. This hold will prevent her from moving, escaping or scratching you.