I don't have a RD88 but some time ago I was interested in it and downloaded some of the manuals. One of them is a detailed one concerning detailed parameters:
Here's what I see but you will have to test yourself and see if that's the actual case:
Here’s how to edit settings for the currently selected scene.
1 . Press the [MENU] button.
2 . Use the cursor [ ] [ ] buttons to select “SCENE EDIT,”
and press the [ENTER] button.
3 . Use the cursor [ ] [ ] buttons to select the item that you want to edit, and press the [ENTER] button.
4 . Use the cursor [ ] [ ] buttons to select a parameter, and use the [DEC] [INC] buttons to edit the value.
From there it seems you have to select IFX (for insertion effects) and see what type is selected, probably it's the "Delay -> Trem" (delay + tremolo) that has a parameter called "Trm Hz" (frequency of the tremolo).