You can switch between channels on a vidIQ account by going to “Channel settings”. Then you need to remove the current channel clicking on “Remove this channel from your account”. Now are able to add a new one.
How to remove a channel from a vidIQ account:

Log into your vidIQ account on https://app.vidiq.com/
Click on your profile on the top right of the page
Select the channel you would like to remove
Click the red 'REMOVE THIS CHANNEL FROM YOUR ACCOUNT'
Confirm and you're done!
How to add a channel. Way 1:

Log into your vidIQ account on https://app.vidiq.com/
Click on your profile on the top right of the page
Now select the green '+ ADD CHANNEL' button
Select your account
Click 'Allow' and then 'Continue'
How to add a channel. Way 2:

Authenticate your YouTube channel
Select your account to the right side of the site
Click 'Allow' and you're all set.