In the Clients tab, Edit the client you wish to assign the package to.
Open the 'Package/Membership/Prod' tab, click on the '+ Add Package', button to select the appropriate package.
A confirmation window will pop up to confirm adding the package. You can customise the package particulars if you wish.
If clients are allowed to make up for a session/class that they cancelled/missed, you can create a Make-Up Package that would compensate for the cancelled/missed appointment.
To do so, follow the steps to create a regular package, and set the number of sessions or classes to the number that the client has missed. You can set the price to $0, or a different amount if you want to charge them. Once created, you can then go ahead and apply the package to their account.
Alternatively, if their package hasn't run out yet, you can manually adjust the total number of credits available in their 'Packages' list: