Dyson cyclone V10 User Manual page 12

Before replacing, check that your brush bars are completely dry. Refit your
small brush bar first, insert the round end into place and then push down the
octagonal end until it clicks into place.
Reattach the end cap to your main brush bar.
Slide your main brush bar into the cleaner head. Push the end cap back
click
down to the closed position.
Turn the end cap lock 180° clockwise to the locked position. Check that the
fastener and brush bars are all securely in place.