Select the cells containing the full names
Go to the Data tab
Click Text to Columns
Choose Delimited
Click Next
Select Space as the delimiter
Click Next
Choose the destination cells if needed
Click Finish
Use the formulas `=LEFT(A1,FIND(” “,A1)-1)` for first name and `=RIGHT(A1,LEN(A1)-FIND(” “,A1))` for last name
Use `=TEXTBEFORE(A1,” “)` for first name and `=TEXTAFTER(A1,” “)` for last name in newer Excel versions
Use Flash Fill by typing the first name in the adjacent column and pressing Ctrl+E
Use Flash Fill again for the last name in another column