How to change hash checksum of a text file without changing the contents in the file
Hash functions are deterministic, same input same output. Without a modification, you cannot change the result.

The hash of a file is used at least for the integrity check. If you were able to change it without modification the integrity check will have problems.

Note: as mentioned by Ella Rose in the comments if a keyed-hash message authentication code HMAC is used then changing the key will result in a different hash-based message authentication code.