You can reassign contacts one of two ways:
If it's a clean reassignment - all leads from one agent to another -- make the current owner inactive in their employee record. When you save the update to "Active - No" the system will alert you that the agent owns leads. You can then select the new agent from a pick list, and save.
If it's more than one agent you are distributing to you will want to 1) create Contact Groups for this, then 2) update the Sales Executive using Contact updater.
First, contact groups - please see this article: https://focus3inc.zendesk.com/hc/en-us/articles/212086866-How-do-I-create-a-Contact-Group-
Then, using contact updater, (Admin, Contact Tools, Contact updater) pick the group you want to reassign and then select the new Sales Executive.