If you are wanting to keep the word "Chapter" & numbering (just deleting the colon) you would want to modify the numbering style you are using (i.e., Heading 1) via the "Define New Number Format" dialog box. To get that (1) right-click on the Style in use to get the Modify Style dialog box; (2) Click on "Format" then "Numbering" to get the Numbering and Bullets dialog box; (3) click on "Define New Number Format" to get the dialog box; then (4) modify the "Number format:" to what you want.

You indicated that it now reads Chapter 1: etc. If that is true, that is what appears in the "Numbering format:" area. Simply change it to "Chapter 1" [without changing the default "1"] (or, however you want it to appear as). Click "OK" out of the menu's.

Hope this helps...