Like lowering a hem, the amount of seam allowance that exists is going to determine whether or not you can let out a seam. However, sometimes a small change can make a lot of difference. If you can restitch a seam 1/4” inside the current seam on both sides of a garment, you’re actually adding 1” to the diameter of the garment. The main thing to remember is to taper the new seam into the old. In other words, if you want to let out the seams on a dress or blouse, begin stitching on top of the old stitches below the sleeve seam and gradually taper your stitches into the seam allowance over perhaps 1” of stitching.
If you are letting something out clear to the hemline, remove the hem along the seam area, stitch the seam, remove the old seam stitches, press the seam flat and repair the hem.
Taking in a seam is a lot easier because you have more to work with. Begin by putting the garment on inside out. Place pins along the lines where you want new seams. Shaping seams can be lengthened the same way. Carefully remove the garment. Shift the pins to make side seams match.
To be sure the garment will fit the way you want it to, first stitch the seams with long basting stitches. These can be machine basting stitches or running stitches done by hand; the idea is to use easily removable stitches. Try the garment on again right side out. When it’s right, stitch by tapering out from the existing stitches as described for letting out a seam. Trim or finish the seam allowance edge, copying the existing treatment as closely as possible. Press and repair the hem if necessary.