I'm looking for a function where I want to replace the genre for some tracks. For example if I have a lot of tracks with the genre House Tech and want to replace this with House Techno, where can I find this in the program? I'm looking for a function "find" and "replace all", or just "replace".
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 ActionReplace for field GENRE where you replace House Tech by House Techno. Make sure you have Matches Word checked:
Just FYI, the actual genre I think you are looking for is Tech House.
There is no official House Tech or House Techno genre, Tech House and Techno are both very different.