On the details page for an Investment, there is an Investment Type drop-down list. If the transactions are all of a type that the particular type supports, you can change the type.
There is no setting to change the wording from Mutual Fund to something else. This is not to say that there might not be a file that could be edited, but it would not be straight forward.
Consider Other as the type of some investment that is not a stock and not a mutual fund. This is good for PTPs, for example. . Don't select Bond, even if it is a bond fund. For a Bond fund, use Mutual Fund. You may not be able to change an existing
investment to Other.