For editing the groups a user is currently part of on the system, you can use the below steps:
Find the user, this can be either within a list of users, or as a search, as below:
You can then view the details by clicking the result for the relevant user and access the group options from the 'Groups' tab:
You can then expand the group categories to view all groups, and select/remove those for the account, as needed:
You can then use 'Save' to save any changes.