Hello, I am a solicitor with 20 years experience. I will try to answer this.
I assume she is an assured shorthold tenant, as most tenants are.
I will also assume that you have given valid notice and that the fixed term of her AST has expired.
If she has not in fact not left you can not enter and change the locks on her as this would be an unlawful eviction for which you could be arrested and prosecuted.
You would need to go to court to get a possession order before you could evict her.