Many questions are asked about pruning lemon trees especially if it has become to large for the space.
The answer is simple. In spring when the fruit is ripe always pick the fruit with a pair of secateurs. Harvest the fruit by cutting the small branch that the fruit is on. Cut it back to the nest branch or leaf. This way means that each time you pick a lemon throughout the summer a small piece of the tree will be removed. Later in the summer it will be possible to remove some longer branches as there will be less fruit in the tree.
Occasionally there will be longer branches to remove. Cut these back to the junction of another branch to fit the shape of the tree required.