How to blur the background in photoshop?

Gaussian blur will bleed into the non-selected area.  The Blur Gallery filters don't have that problem.

Original

Gaussian Blur. You can see a fringe bleeding out from the beige square.

Field Blur (from Blur Gallery).  No fringe

Fastest is :

Open discover panel

• Select quick actions

• Choose backround blur

Photoshop will do the job for you

Briefly: Select > Subject. Then reverse the selection, apply a Layer mask, fine tune. Apply a Gaussian blur.

A screen shot of the image posted here might help us see the better.

Select portion that you want to blur using any selection technique then blur it using Filter > Blur > choose one like Gaussian Blur filter. You can convert layer to Smart Object layer to apply non-destructive blur.