You may kindly refer to this forum topic as the texts can be changed through "Language Override" option
Where to add new fields...
For adding new field accordingly, You can replace the files attached :
1) default.php > unzip and extract in \templates\ja_space\html\com_content\article folder
2) project.xml > unzip and extract in \templates\ja_space\etc\extrafields folder
Would suggest to take backup of your current files before performing aforesaid steps, in case rollback required whilst facing any issues e.t.c
The above steps will result in adding of new field (ref: screenshot) for which text label can be changed accordingly through "language override" option