Short answer
You can change the country or region in the app, but this may affect your current plan.
Before changing country
Please note:
your current subscription may be replaced;
previously purchased data may not be preserved;
in some cases, the eSIM may need to be installed again.
Steps
Open the app.
Open the country or destination selector.
Choose the new country or region.
Review the available plans and confirm the change if needed.
If the eSIM does not work after changing country
Make sure you have arrived in the new destination.
Check whether the eSIM needs to be installed again.
In the app, go to Settings → Get Link to Install eSIM if needed.
If you changed the country by mistake, contact support.