I've not heard of Sibelius free but this is from the Sibelius 5 reference manual under the heading 'Beam and Rest Groups'.

"Sibelius automatically groups beats appropriately for each time signature you create. You can, however, change these settings for yourself by clicking 'Beam and Rest Groups' in the 'Create' > 'Time Signature' dialogue, or you can change the beam groups in existing music in the 'Notes' > 'Reset Beam Groups' dialogue.

I take this to mean that you either look for the top menu item 'Create', select 'Time Signatures' and then select 'Beam and Rest Groups', or select 'Notes' and then 'Reset Beam Groups'.

My guess would be that the second one should suit best. It might be a good idea to highlight the notes in question first.