Open both workbooks. There are several ways to do this. Probably
depends on how much data you're copying.
If it's only a few cells, open both spreadsheets. Put an = sign in the
cell you want to transfer the data to (target sheet), then click on the
cell you want to copy (source sheet), then click on the enter button
(checkmark) on the entry line in the target sheet. The 2 cells will be
linked.
If you want to copy a large area of a spreadsheet, click and drag over
the data you want to copy in the source sheet. Click on Edit, Copy. Go
to the target sheet, highlight where you want to data to go, do an
"Edit, Special, Past Link". The 2 spreadsheets will be hot linked.