There are sewing tools to remove stitches. They are called seam rippers. Seam rippers come in a variety of shapes and forms. Some are safer than others. Most sewing machines come with the type of seam ripper that is shown here.
It is not advisable to use razor blades or sharp knives to removes stitches. This type of tool will not only cut the thread you want to remove but also will cut through the fabric easily.
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