On the app click on 'Settings' in the lower right corner, then 'Sites' in the middle:
Then begin typing the name of the site you'd like to switch to, which will filter the list and you can then select the desired site:
If you are a member of more than one jobsite which uses the Converge platform you will need to navigate between them.
To do this in the platform select the arrow in the top left corner:
You are then see a drop down with all the sites you are part of. Either select from here or type in the site you need.