N.H. Rev. Stat. Ann. § 296:8

Such corporation, by unanimous vote of all the shares represented at any meeting called for the purpose, or by the written consent of all the stockholders filed with the clerk, may increase or diminish the number of its shares, and thereby increase or diminish the par value thereof; but the capital stock shall not be increased or diminished thereby, and the par value of the shares shall not be fixed below $25.