You can edit the client's milestones from their profile!
Go to Clients > search client name and click on their profile
Click the three dots under the client profile picture > Edit Client
Scroll to bottom of the pop up to "Original Milestone"
Edit milestone then click Save
Note: When editing the milestone, keep in mind that this is the number of classes taken before joining Arketa -- contributes to the client's milestone.
For example, if a client shows 1 milestone from a booking in Arketa, then you add 5 to the "Original milestone", then the milestone would be 6.