Adding "Unit" into my heading 1

[drew345]

I currently have heading 1 numbers and heading 2 numbers like this:

1.
1.1
2.1
3.1
2.
2.1


2.2
2.3
3.
3.1

I want to put the word "Unit" in front of all heading 1, so that it looks like this:

Unit 1.
1.1
1.2
1.3
Unit 2.
2.1
2.1
2.3
Unit 3.
3.1

But when I modify the numbering for heading 1, somehow heading 2 also gets modified, and worse, heading 2 stops being level 2 numbers like this.

Unit 1.
Unit 2.
Unit 3.
Unit 4.
Unit 5.
Unit 6.
Unit 7.
Unit 8.

So I go into heading 2 and remove the "Unit" from it. But then it becomes some totally independent level 1 numbers, instead of level two numbers like it should be:

Unit 1.
1.
2.
3.
Unit 2.
4.
5.
6.
Unit 3.
7.

So what I am looking for is this below. Any idea how I can achieve it? Seems obvious but I can't seem to get it.

Unit 1.
1.1
2.1
3.1
Unit 2.
2.1
2.2
2.3
Unit 3.
3.1