Step 1: open up your Photoshop and the picture you want to change the background colour.

Step 2: click the quick selection tool here and use that tool to select the person. if the selection goes out of the person, subtract the selection using this  you can find it at the top of Photoshop

Step 3: after that, at the top of photoshop click refine edge. here this should pop up.

click the smart radius. after that adjust the radius. mess around with it. after you’ve finish click ok.

Step 4: right click on the picture, select inverse like this  after that click layer>new fill layer>solid color. like this

Step 5:  After that simply click ok without doing anything. After you click ok. this will pop up.

Step 6:  Choose any solid colour you want to fill your background. after you’ve finish click ok.

Step 7:  you’re done! click ‘save’ or ‘save as’.  make sure that you change the format into JPEG.