When you have made changes to your PSC register and it has been sent to Companies House, there is nothing more that you can do for that individual until Companies House accept or reject the current filing.
Note: You will also not be able to file a confirmation statement whilst this is pending.
Here I have made changes for the PSC Patrick Brown:
You can see the status of the PSC is pending.
If I try to make any changes to the PSC Patrick Brown I get the message below: