how to change the mode between the classic and modern mode?

Hi @Cici L P CHEN

Based on your description, I understand that you want to switch a Power Apps canvas app from the modern controls/themes experience back to the classic experience.

In a canvas app, there is no separate “classic mode” button for the whole app runtime. You can check the modern controls setting by opening the app in Power Apps Studio, then going to Settings > Updates > New and turning off Modern controls and themes if that option is available in your environment.

If your main concern is that classic themes are no longer visible, go to Settings > Updates > Retired and enable Keep classic themes. Microsoft notes that when modern controls and themes are enabled, classic themes are removed from the command bar unless this retired setting is enabled.

Also, please note that modern controls and classic controls are separate control sets. Existing modern controls may need to be replaced manually with classic controls from the Classic or Classic icons section.

Reference:

Use modern themes in canvas apps

Overview of modern controls and themes in canvas apps

Thank you.

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