bbPress.org

idk havent tryed editing the submit button for certain forms yet

but for profile its

#bbpress-forums #bbp-your-profile fieldset.submit button {
float: right;
}

for reply form its

#bbpress-forums .bbp-reply-form fieldset.bbp-form button {
float: right;
margin-top: -120px;
}

you can try that , other than that try doing what stephen said

You should look up some CSS resources and use your browsers ‘inspector’ to look at how to make CSS changes to your theme, use your favourite search engine to find these.