sewing shoulder seams

My front and back shoulder edges have uneven number of stitches (back has 13 and front 10 ) so the back shoulder is wider than the front one (the yarn is bulky). How do I sew them together ? Thank you,

Welcome to KH!
Is this correct according to the pattern? Usually the front and back shoulders match up because you have to seam them. Which pattern are you making? Can you give us a pattern link or a pattern name?
Is the shoulder sloped or is there a single bind off for the shoulder? In other words, did you bind off in steps or in one row, straight across?

Thank you for your quick response. I am not experienced and might have
misread the pattern, but all the other numbers (stitches) turned out as
expected. This is what I am making (I just found this online version of the
The shoulder is sloped, three BO steps for back and two for front.
For medium sized sweater, the shoulder shaping is using 13 stitches for
back and 10 for front.
Thanks a lot.

Yes, that’s all correct. You just have an uneven number of sts. I’d line up the armhole edge and pin it in place, then line up the neck edge and pin in place. Next pin the center of the shoulders together. There’ll be a bit more fabric on the back but since knitting is stretchy, you should be able to align fairly well, then seam in place.
A back stitch seam with a thinner yarn will work. Mattress stitch seams while neat would require seaming 2sts from the back to one stitch from the front in 3 different places and I think it wouldn’t be as neat a finish.

Thank you so much. I will do that. Thanks!