s,
As I (mis)understand it, you wish to have the left side of the neck path fit into the neck (opening) of the garment path, while retaining a symmetrical shape.
Your first image shows that it is a wee bit too narrow and the right side (as seen by us) seems (more or less) fit the right corner of the neck (opening) of the garment path.
Basically you can choose between:
A) Making the current neck path shape wider while keeping it symmetrical and with the top (and bottom) at the same vertical position(s),
B) Keeping the current neck path shape as it is (symmterical) and moving it (a wee bit to the left and a bit upwards) into position so it fits (in both sides).
B) is a bit simpler, but your mentioning "change an object" rather than "move an object" makes me believe that you prefer A), so the following is based on that:
However, if you just deselect and ClickDrag the leftmost path segment with the Direct Selection Tool by some position on it to snap to the corner of the neck (opening) of the garment path, it will lose its symmetrical shape for at least one reason, maybe two:
If you ClickDrag the path segment to the left and a bit up or down, its end Anchor Points will move up or down, and the top curve will be skewed because the left end of the top curve will be higher or lower than the right corner (the same applies to the bottom which has no consequence because it is hidden);
If the top curve has a central Anchor Point, it will remain in place like the right end and thereby go off centre so the curve will be further distorted.
You can choose to skip the symmetry, which is unecessary if you hide the top of the neck part hidden behind the head part, in which case you could also choose B).
Or you can challenge yourself and learn a few subtleties.
If you wish to retain the symmetry, you can do it accurately, using Smart Guides in the View dropdown, and the Scale Tool in the Toolbox, and the Line Segment Tool which you can read about (and find) here,
https://helpx.adobe.com/illustrator/using/drawing-simple-lines-shapes.html
You can:
1) With the Line Segment Tool ShiftClickDrag (hold Shift and ClickDrag) horizontally between the corners of the neck (opening) of the garment path, either way (Smart Guides will say anchor when you are within snapping distance); holding Shift ensures a horizontal line so you can check that the garment is symmetrical;
2) With the Normal Selection Tool Click the leftmost path segment on top of the intersection with the line from 1); (Smart Guides will say intersection when you are within snapping distance), then ClickDrag left to snap to the corner (Smart Guides will say anchor),
Now you have moved the whole neck to fit the left corner, and you need to widen so it fits the right corner as well:
3) With the neck path still selected switch to the Scale Tool and Click on the fit at the left corner to make it the key point, then (almost as in 2) Click the rightmost path segment on top of the intersection with the line from 1), then ClickDrag left to snap to the corner (Smart Guides will say anchor).
By using the Scale Tool you widen the whole neck path symmetrically.
If you are sure that you have the right fit at the right corner of the neck (opening) of the garment path to start with, you can skip 2) you can use the Scale tool the opposite way to get the fit on the left corner..
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