Click [Your name] on the upper right corner of the chat screen and click [Profile] → [Edit profiles] on the displayed menu to make changes.
Input the name you wish to display in [Display name] and click the [Save] button to save changes.
Note, by checking [Allow your profile to show up in search results],
other users will be able to search your [Display name] for the contact search.
*Your [Profile] can be opened from the pop up screen displayed when you click the profile picture.