How can I change page title in search results page?
Install and enablde Page manager and Panels.
Go to admin/structure/pages, on the line "search-node", click enable, then edit.
Add new variant -> Create variant -> Choose any suitable layout -> Continue twice.
On the final page in the Wizard, expand "Substitutions". You should see two keywords in there, copy "%keywords:raw" to the empty "Title" field above
Below the keywords, click a cog in any suitable region, in the popup click Widgets, and select "Search form".
Repeat step 6 for "Search results".
Click "Create variant", followed by "Update and save".


Take variations for choosen layout into account, and your configuration should now look something like this