How to change UI texture

I’m still very new to this and i was wondering how do you change UI elements such as the window and buttons to be my own texture

Check out Data/UI/DefaultStyle.xml with text editor; it references Textures/UI.png.
The ‘User interface’ section goes into more detail: https://urho3d.github.io/documentation/HEAD/_u_i.html

If your UI texture doesn’t align exactly with the default texture (size and placement of elements) then you’ll need to edit the DefaultStyle.xml definitions to suit, or write a new style sheet for it that includes accurate image rectangles and suchlike.