To my knowledge there is no specific search and replace functionality for replacing icons
The procedure described below worked in Mendix 3 version. Don't know for sure if this also works in the newer versions.
1) delete the old icon from images (please remember the icon name) you first get a warning about on which forms the icon is used 2) Import the new icon that should replace the old one 3) Rename the icon with the (exact) name of the old (deleted) icon
all forms/buttons should now automatically be updated to new icon
Already filled a feature request for thuis a while ago.
For now René's answer is the way to go.