How I would change it would be to make it my personal off-day.

【翻译】我改变这一天（指星期日）的方式是，让它成为我的私人休息日。

【分析】本句结构有点特殊，句子成分分析如下：

How I would change it （主语）would be （系动词）to make it my personal off-day.（表语）

1.   How I would change it： 这是主语从句，表达方式、方法。我们可以用 The way I would change it 替代它而意思保持不变。

2.   would be ：系动词。would 语气婉转表推测。前面所有would 的用法都是表达“语气委婉”。

3.   to make it my personal off-day：动词不定式作 would be 的表语。make 接复合宾语。

既然是主系表结构，为了理解，我把句子简化一下：

The way to change Sunday is to make it my personal off-day.

改变星期天的方法是让它成为我的私人休息日。

把本句理解为“主系表”结构，其本质是：

主语从句的内容跟不定式的内容是对等的，“主表”可以颠倒。也就是说：

The way is to do sth. 

To do sth. is the way.

例如：

The best way is to go at once.

To go at once is the best way.

我们回到网友的句子：

How I would change it would be to make it my personal off-day. =

The way I would change it would be to make it my personal off-day. 

To make it my personal off-day would be how I would change it. =

To make it my personal off-day would be the way I would change it. 

How I would change it would be to make it my personal off-day.

【翻译】（意意）我将会让它变成我独有的休息日。

 前面五个词是主语从句。would be虚拟语气。 

（个人观点）

How I would change it would be to make it my personal off-day.

参考译文：我将如何改变它将会使它成为我个人的倒霉的日子。

主语：How I would change it

谓语：would be to make

宾语：it

宾补：my personal off-day

off-day 恐怕是day-off之误吧