Thanks for taking the time to post. I don't think we do those handsets so I'm not 100% how you go about doing this but I'll try my best for you.
On my phone for example (Galaxy S5) I just click on 'Me' in the Contacts book and it opens up the contact just like any other. At this point you have the option of changing the number. I've also included a link to LG's support site here. This may provide the answer you're after.
Hope this helps. Please let me know how you get on with this.