Currently, if you have a PowerApp and you want to switch it from one owner to the other, it's not possible to do that from the UI. There is a pretty simple solution for it though.

PowerApps <3 PowerShell

Make sure to download the PowerApps PowerShell module from here.

After that, you can import the module & add a PowerApps account just like the blog post from the link above describes.
To change the owner, just use the Set-AdminPowerAppOwner cmdlet!