Pull the hood up from the bottom and see if you are lucky enough to be able to fish it out. Check shimano's tech doc site for the DM, but probably won't be there for that model. Newer models are similar and will give you some ideas.
https://si.shimano.com/pdfs/dm/DM-ST0002-05-ENG.pdf
Top of hood only lets you remove the lever. Though I'm not sure about your older model.