B is indeed a mileage-upgradable fare class on KQ.

1. Look up your reservation from either the Kenya Airways, Air France or KLM (illustrated) websites while logged in with your Flying Blue credentials.
2. Check to see if your booking displays a "Check your options for upgrade with miles" (or equivalent) button.
3. If so, explore your options online.
4. If not, call Flying Blue.

Alternately: check all your upgrade options at https://www.kenya-airways.com/prepar...kq-upgrade/en/