Hello,
in order to edit the default contact page form fiels labels, you have to just simply create a translation file with Poedit and follow the guide here.

http://www.themes2go.xyz/manuals/lifecoach/knowledge-base/translating-the-theme/

Inside our theme, we’ve added the possibility to edit text strings via Poedit so every user can translate or change text parts in an easy and fast way without coding.

Thanks for new information, it is very clear that you need to edit the fields.
Actually, what you have to do is to use Contact Form 7 instead (provided with the theme) and I’ll now explain you why:
The contact form plugin provided with the theme is an alternative solution, a “Plug And Play” system without settings, for those who doesn’t want to edit fields and stuff, avoiding the mess of configuring Contact Form 7.
For its simplicity, it doesn’t have settings like changing fields (otherwise it would have been messy like CF7) but if your needs are to edit the contents of the form, you’ll then need to use the more professional solution (Contact Form 7) instead of the simple Contacts plugin.
I hope my explanation was clear, if you still have any doubt please let me know.
Regards
Igor