1.  rename your web.config file web_4.5.3.config

2.  unzip the DotNetNuke 4.5.5 solution over your currently installed 4.5.3 solution

3. rename release.config  web.config

4. copy your connetion strings from  web_4.5.3.config to web.config

5. copy your machine valadation key from web_4.5.3.config to web.config

6. copy your machine decryption key from web_4.5.3.config to web.config

7. if you hve made any changes to your web config file from your 4.5.3 install make those changes in the new web.config file.

8. open IE and go to your portal.  It should update

hope this helps

Mike