I've imported a table and some of the rows have blank values

I want to update the empty values of CaseID with the preceding value so in this example all the NULL CaseID will be 122303.
I don't mind about the other columns its just the CaseID I want


INC CaseID Status
N 122303 Open: Investigation
NULL NULL NULL
NULL NULL NULL
NULL NULL NULL
N 122331 Open: Investigation


Any suggestions on some SQL to do that ?

Thanks

Nick