Signal servers do not possess any information about groups or your membership in them. Only another member of the group can refresh group info for your phone.
Send a one-on-one message to another member of the group and ask them to send a couple of messages in the group or to update the group name or avatar from their mobile phone. This should refresh the group information on your device as well.
Steps to edit the group name or photo from the mobile phone
- Open the group conversation and tap the group name.
- Tap the edit icon or Edit in the top corner.
- Choose a new name or photo.
- Tap Save or Update.