5 CFR §1651.4

1. (a)Change. To change a designation of beneficiary, the participant must submit to the TSP record keeper a new TSP designation of beneficiary meeting the requirements of § 1651.3 to the TSP record keeper. If the TSP record keeper receives more than one valid designation of beneficiary, it will honor the designation with the latest date signed by the participant. A participant may change a TSP beneficiary at any time, without the knowledge or consent of any person, including his or her spouse.
2. (b)[Reserved]
3. (c)Will. A participant cannot use a will to change a TSP designation of beneficiary.