bbPress.org

then you need to amend form-topic and form-reply

to amend

<button type="submit" tabindex="<?php bbp_tab_index(); ?>" id="bbp_topic_submit" name="bbp_topic_submit" class="button submit"><?php _e( 'Submit', 'bbpress' ); ?></button>

to something like (untested)

<?php if ( bbp_allow_revisions() && bbp_is_topic_edit() ) : ?>
<button type="submit" tabindex="<?php bbp_tab_index(); ?>" id="bbp_topic_submit" name="bbp_topic_submit" class="button submit"><?php _e( 'Submit', 'bbpress' ); ?></button>
<?php endif ; ?>
<?php else : ?>
<button type="submit" tabindex="<?php bbp_tab_index(); ?>" id="bbp_topic_submit" name="bbp_topic_submit" class="button submit"><?php _e( 'Update', 'bbpress' ); ?></button>
<?php endelse : ?>