Go to the size chart you want to edit, then hover to your size chart and click "Edit table."

From the Size table editor, open the Editor Mode dropdown at the top of the Size table panel and choose "Advanced mode."
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Right-click anywhere in the table, and choose the "Merge cells" option.
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Choose how many cells to merge, for example "Merge with right 2 cells" or "Merge with bottom 1 cell."