Can a tenant change the locks without the landlord's permission?

Your tenants do not have a right to change locks if the locks were not broken and there were no issues of prior tenants possibly having duplicate keys.  Typically landlords will change the locks once tenancies change (tenants move out, new ones move in).  If all of this applies, you have a right to a key immediately. If not, they have effectively created a problem for you safety wise, in case you need to get in to fix something or prevent additional damage.  You need to get the key immediately.

If they had asked you to replace the locks for some reason like issues mentioned above, and you promised to do so but have not, your tenants can fix it themselves and deduct the costs from their rent.