very good question, right, well download the latest Build of photopost (5.1) and make sure you edit the config-inc & config-int.php files and edit/enter the relavant information i.e your database settings, urls etc as you would have done with 4.6 then most importantly :
backup your existing photopost database and your data directory
Just incase anything should go wrong (which i doubt) you'll be safe
There will be upgrade instructions included with the install which you can refer to also as you go.
Upload the new 5.1 files to your photopost directory overwriting files is ok, when finsihed uploading you must then alter the permissions (CHMOD) to your photopost main php files (index, config-inc, pp-inc) etc to 777 also the data directory (and all subdirectories) to 777 to.
Then after that point your browser to the "upgrade.php" file which should reside in your main photopost directory and follow on screen instructions and all should be ok.
If you something should go wrong, re-check your permissions and run the upgrade again (but only to step 2, don't goto step 3 second time around) hopefully you won't get any problems during the upgrade.
Follow these steps and i'm sure you'll be fine, if your not sure about this you can apply for the upgrade service from Chuck, ('Omegatron' on the forums)
Please report back on how you get on or if you run into any problems.