If you are unable to retrieve your repaired computer within a year, (please be advised) we will dispose of it at our own discretion.
If you don't pick up your repaired computer in a year, we will dispose of it.
このような文になります:
If you are unable to retrieve your repaired computer within 1 year, (please be advised) we will dispose of it at our own discretion.
unable to retrieve = 引き取りに来られない
within a year = 1年以内に
at one's own discretion = 自己の判断で(行動する)
dispose = 処分する
この場合に「弊社」はweやourとして使っています。
また、( )のところに、please be advisedとかplease understand (ご了承ください)というフレーズも加えるとより丁寧になります。
もうカジュアルな言い方なら、
If you don't pick up your repaired computer in a year, we will dispose of it.
も場合によって使えます。