Once you have created a working pattern for a staff member you are unable to edit it once you have assigned it to a staff member.
If a staff member changes their working pattern once you have assigned a working pattern to them, you need to go to that staff member's Profile and click on More and then on working patterns tab and then choose Change working pattern
A new dialog box will pop up and you can click on the drop down menu to choose the New pattern for that staff member and then click on Apply pattern
If the Working pattern you require doesn't exist, then you will need to create a new working pattern for that staff member. Here is a link to the Knowledge Base article on how this is done https://staffsquared.uservoice.com/knowledgebase/articles/418244-how-to-create-a-working-pattern.