Changing a user's default 'Preferred Technician' setting (for when this user creates a new customer record) is easy.
FROM THE WEB APP
Step 1: Go to Settings > Team Members
Step 2: Select the user you want to edit.
Step 3: Check the box "Set this user as the preferred technician when this user creates a new client..."
Step 4: Save
FROM THE MOBILE APP
This setting can only be changed by logging into the Web App at this time.