To disassociate file types from ACDSee, please try the following steps:

1.    In ACDSee, click Tools | File Association Settings

2.    In the four format sections, deselect the checkboxes of the file formats you would like to disassociate with ACDSee Ultimate for image formats, RAW formats, media formats, and archive formats.

3.    Click Save

4.    Close ACDSee 

You can then re-associate the file type to the default program in Windows File Explorer:

5.    Right click the file type, e.g. PNG, and then click “Open with..” in the fly-out menu

6.    In Windows 7 and Windows 8, click “choose default program…”; in Windows 10, click “Choose another app..”

7.    Ensure the option “Always use this app to open this file type” is checked and select the application that you want to open this file type.

The file type will now be associated with the program you selected in step 7.