In repository folder find .git folder, there will be file with name "config" (without extension).
Check there [remote "origin"] section, if url starts with https, then you should change it to ssh-link (can be found on your repository's github page, looks like "email@example.com:username/reponame.git").
To check, if your SSH-keys is used properly, you can open GitBash and enter following text:
ssh -T firstname.lastname@example.org
Keys is always pair id_rsa and id_rsa.pub, you can't change keys in PhpStorm, you can only generate and upload public keys to github from each workplace.