If the variations are known, you should be able to use two (or more) nested replaces to get it to your desired format - for example like shown below-- see if the path exists and what the fixed up path would be
SELECT dirpath, replace(replace(dirpath,'/','\'),'\\','\') FROM theTable;
-- replace with the new path
declare @newPath varchar(255)='D:\Dir1';
declare @oldPath varchar(255)='C:\Dir1';
UPDATE theTable SET
dirpath = @newPath
where
replace(replace(dirpath,'/','\'),'\\','\') = @oldPath ;