Hi there,
I'm Amr, an independent advisor.
After opening the desired photo, click on the "Edit image" button at the top of the page and select the "Crop" tab.
From there, you can use the slider to straighten the photo as desired.
Also, If you wish to use the older version of the app which is now called Microsoft Photos Legacy, you can download it from here
https://apps.microsoft.com/store/detail/microso...
I hope this is helpful. If you require any additional assistance, don't hesitate to respond to this.
Best Regard
Amr