I need to change a multiple choice-text field to a text field and there is no option to change field type

There is a radio button in the field properties to change the input toe to user input.

But rather than do that, if you really do want to control the choices available le to Used, you can create a new table of choices and the key field to be the choice value. Then copy the existing choices onto your clip board, and use import export to import them into your choices table.

Then make a relationship to your data table. Then you can have unlimited choices and also users will be able to search for their choice