You can't split an individual cell, but you can make it appear as if a cell has been split by merging the cells above it.
Merge cells
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Select the cells to merge.
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Select Merge & Center.
Important: When you merge multiple cells, the contents of only one cell (the upper-left cell for left-to-right languages, or the upper-right cell for right-to-left languages) appear in the merged cell. The contents of the other cells that you merge are deleted.
Unmerge cells
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Select the Merge & Center down arrow.
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Select Unmerge Cells.
Important:
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You cannot split an unmerged cell. If you are looking for information about how to split the contents of an unmerged cell across multiple cells, see Distribute the contents of a cell into adjacent columns.
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After merging cells, you can split a merged cell into separate cells again. If you don't remember where you have merged cells, you can use the Find command to quickly locate merged cells.
Source: Microsoft