You 'tweak' it exactly like any other formula, by passing the arguments required.
Example: with dates in Column A, Values in Column B, you might write something like:
Formula:
=XIRR(B1:B5,A1:A5)
Now to adjust this for an 'unknown' number of dates / values, you might change it to something like:
Formula:
=XIRR(OFFSET(B1,0,0,COUNTA(A:A),1),OFFSET(A1,0,0,COUNTA(A:A),1))
So if you are failing with your formula adjustments, then you're getting the OFFSET function wrong.
Why not post an (anonymized) example of your worksheet structure, and what you are trying, so that we can offer more meaningful examples?
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