After plaintiff took possession, the parties amended their lease agreement. An oral amendment to a written lease *580is ■ effective if the oral agreement “has been acted upon and become executed.” Johnson & Dix, Inc. v. Burlington Mutual Fire Insurance Co., 123 Vt. 488, 489, 194 A.2d 401, 402 (1963). In this case, after plaintiff took possession, he requested , a modification in the heating facilities, a change that was agreed to by defendant. The change effectively postponed the date of installation of the heating facilities because the overhead blowers plaintiff desired had to be specially ordered. In such circumstance, defendant was entitled to a reasonable period of time within which to install the desired units, Hayden v. Hoad-ley, 94 Vt. 345, 348, 111 A. 343, 345 (1920); 1 Corbin on Contracts § 96, at 412 n.15 (1963), and plaintiff was not justified in withholding his rental payments and condominium fees during this period while remaining in possession of the premises. The trial court correctly concluded that plaintiff’s failure to make the rental payments constituted a breach of the lease agreement. See Northern Terminals, Inc. v. Smith Grocery & Variety, Inc., supra, 138 Vt. at 396-97, 418 A.2d at 27 (distinguishing recovery of damages for breach of covenant from excuse from obligation to pay rent because of constructive eviction); Legier v. Deveneau, 98 Vt. 188, 190, 126 A. 392, 393 (1924) (“unjust to permit the tenant to remain in possession and escape the payment of rent by pleading a state of facts which, though conferring a right to abandon, had been unaccompanied by the exercise of that right,” where tenant had raised defense of constructive eviction).