Once you and your partner are connected in Cupla, you can update their name at any time. Here is how:
Tap the menu icon (☰) in the top-left corner of the Home screen. If there is no ☰ on your Home screen, tap More in the bottom-right corner of the screen instead.
Tap your partner's name or profile circle (top right).
Tap the pencil icon (top right).
Update the name and tap Save.
The updated name will appear throughout the Cupla app.
If your partner has not joined you in Cupla yet, the name you entered for them when you set up Cupla cannot be changed yet. Until they join, the space for them at the top of the menu shows Invite [name] to Cupla. When they sign up from your invite, Cupla welcomes them by that name and uses it as their own name, and they can tap Update my name on that screen to choose a different one. Once they have joined, you can change the name you see using the steps above.
Please note, this only changes how your partner's name appears on your device. It does not change their account name.