Any signer may opt to change the signatory to someone else via the Other Actions menu at the top right.
1. Visit the agreement link as the original signer
2. Click Other Options at the top right
3. Click Reassign
Then simply enter the name and email of the new signer and click Reassign.
The new signer will automatically get an email notifying them that the document is now awaiting their signature. And the person who created the agreement will also receive notification of the reassignment.