Once you have added the key signature you will find that all your Bs are now shown B naturals as adding the key signature does not change the pitch of any notes. You can change them to Bbs by right clicking on one of the B naturals and selecting similar, same pitch and then press the down arrow. Repeat for other octaves if necessary.
> selecting similar, same pitch
To be exact: it is not "Select / Similar" but "Select / More..." and not same pitch but "Same note name".
Otherwise you select only e.g. the Bb3 and not Bb4 or other Bb's.
Comments
Do you mean the key signature for F major?
If yes: open the key signature palette and drag the signature to measure 1.
In reply to Do you mean the key… by HildeK
Once you have added the key signature you will find that all your Bs are now shown B naturals as adding the key signature does not change the pitch of any notes. You can change them to Bbs by right clicking on one of the B naturals and selecting similar, same pitch and then press the down arrow. Repeat for other octaves if necessary.
In reply to Once you have added the key… by SteveBlower
> selecting similar, same pitch
To be exact: it is not "Select / Similar" but "Select / More..." and not same pitch but "Same note name".
Otherwise you select only e.g. the Bb3 and not Bb4 or other Bb's.