You can revoke a financial power of attorney at any time while you are competent. The revocation must be in writing, and a copy of the writings sent to the agent and to anyone the attorney-in-fact has dealt with or is likely to deal with on your behalf. It is critical to give notice to both the agent and third parties who your agent might deal with on your behalf, such as banks, insurance agents, financial advisors, etc. because without that notice they may legally continue to respect the power of attorney. Merely signing a revocation will not end the authority as a practical matter for anyone who has not received notice of the revocation.
For more information, see Uniform Power of Attorney Act, W. Va. Code §§ 39B-1-101, et seq. (2015).