Irfan,
Please follow the below steps to change the Portal browser tab:
1. Log into the API Portal CMS (<portalhostname>/admin)
2. Sitebuilder > Pages
3. For the page.xml file at the bottom, hover over the Layer 7 icon all the way to the right, and click into 'Properties'
4. Under the 'Custom' tab, change SEOTitle to whatever you'd like
5. Scroll down to the bottom and hit 'Apply Custom Detail Changes'
6. Refresh the CMS page
7. On the same page.xml line, a box with a green arrow should've appeared now, please click 'Publish'
After this, you the tab should've changed to what you typed in step 4. I hope this helps.
-Alec Daniello
APIM Support