The simplest way to de-activate the 2nd bike is to go to More > Settings > Bike 2. On top of the page that pops up, there is an on/off switch. Set it to Off and the 2nd bike will be de-activated.
If there is really a need reinstall the app, your purchase won’t be lost. App Store will check and when their system identifies that you had purchased the upgrades before, they will allow you to upgrade without re-charging you.