Hi and welcome!
You can enter "House Tech" in the Filter field on the toolbar which will limit the file lists to the files that somewhere contain "House Tech" — also the files with the proper "House Techno".
To have all the files with "House Tech" in one sequence, you can sort by Genre via a click on the header of the Genre column (or View → Sort by → Genre).
Then simply select all the files with "House Tech" and change the Genre via the Tag Panel on the left.
Alternatively, you can use a Quick Action Replace for field GENRE where you replace House Tech by House Techno. Make sure you have Matches Word checked:
