If a squatter won’t leave, canI change the locks?

First of all this person appears to be what many states refer to as a "licensee". That is someone who was invited into the premises with the owner's or legal occupant's permission (i.e. long term guest). Therefore, although there is no formal lease or even rent being paid, she still must be afforded all of the legal protections that a month-to-month tenant would be granted. This means that you must serve her a notice to quit the premises. If she then refuses to vacate by the specified date given, you will have to file an "unlawful detainer" (i.e. eviction lawsuit). Once the court issues you a writ of possession, then if she fails to remove herself, you can get a sheriff to do it.

You must be aware however, that if this person has attained the status of a legal "tenant" (as oppossed to merely that of a licensee), then only your landlord can evict her since only landlord's may evict tenants. She may have achieved such a status if, for example, the landlord has treated her as a tenant by accepting rent directly from her (not your case), by putting her name or letting put her name on the mailbox/doorbell (apparently your situation), or if you and this person rented the place together and it was clear that both of you were on equal footing, etc.

So, at this point you should consult directly with a real estate attorney in your area who specializes in landlord-tenant matters. They can best advise as to all of this after hearing the specific details of your case. In the meantime, don't attempt any self-help remedies such as changing the locks or removing her belongings. You could find yourself on the wrong side of a lawsuit if you do.

Note: If her behavior rises to the level of being a serious threat to your safety, then contact the police. They then can have her immediately removed from the premises and you can then make application to the court for a protective order to be issued against her.