To process a share buy-back (i.e. selling shares back to the company for a particular shareholder) in Cake,
Head to 'Captable from the main menu on the left;
Go to 'Shareholders';
In the top right corner, click 'Actions';
Choose 'Buy back' from the dropdown menu;
Search for the relevant Shareholder, Share Type, enter the Date of the Buy back, Number of Shares being sold back to the company, and the amount paid and then finalise by clicking Buy back.
[AU Only] Remember that this does not process it with ASIC so you will also need to submit the relevant forms to them.