Open Clients, locate the client and click Actions → View (you can also achieve the same result by clicking the client’s name).
In the client's profile, click Edit client.
Make your changes, then select Save to update the profile.
Australia (AU) only — NDIS clients
On the client profile, on the right hand side panel look for the NDIS details section and select Edit. Enter the client’s NDIS number and Fund management (Agency, Plan managed, or Self managed), then Save.
Want the comprehensive list of editable client fields? Check All Client Fields in the Client field reference.