Either by storing it beforehand:

Slot slot = _skelAnim.skeleton.FindSlot("head");
Attachment prevAttachment = slot.Attachment;
..
slot.Attachment = null;
..
slot.Attachment = prevAttachment;

Or query all attachments from a skin:

List<SkinEntry> attachmentsAtSlot= new List<SkinEntry>();
skin.GetAttachments (int slotIndex, List<SkinEntry> attachmentsAtSlot)

You can also query it by name from the skeleton or from a skin:

Attachment attachment = skeleton.GetAttachment (int slotIndex, string attachmentName);
Attachment attachment = skin.GetAttachment(slotIndex, attachmentName);

or in one method:

skeleton.SetAttachment (string slotName, string attachmentName);

And as always, please have a look at the API reference documentation pages, there you will find all relevant methods:
API Reference - Spine Runtimes Guide