When you have unpublished changes on an active change source, such as in the middle of an IPC update, and if you want to action a manual change due to other literature affecting the configuration data, you may need to revert changes.
Once changes have been reverted, you will be able to modify the change source for the part group to the relevant change source, and make manual changes to the part group, as required. You can then publish this change source, and set the change source back to the one with changes to be made.