How to make a canvas mock up thicker and wrap my image

[benchesters]

Hi, I am reasonably new to Photoshop so please bear with me! I have some canvas art that I need to show mocked up, the size of the canvas is 40inches x 30 inches, I want 1.5 inches each side to wrap around the canvas frame, so my image is 43inches x 33 inches. I have a got a mock up psd file, but it is too thin. Does anyone know where I need to look within this file so make this thicker? Thanks in advance. File attached:

Mod edit: File link removed. Untrusted site.

 

[Mary]

I think you could you Photoshop- Image - Canvas Size and then just add the border size you require.