It really depends what sort of changes you are making.
If you're changing the database definition, then the XMLDB editor (which you should always use to generate the install.xml file) can also generate statements to go in the upgrade.php file - so just put them in there, bump the version number and visit the notifications page, to run the upgrade.
If you're changing capability definitions, message providers, event handlers or any of the other definitions that sit inside the db/ subdirectory, then just bump the version number and visit the notifications page.
If you're changing language strings or styles, then purge caches (site admin > developer > purge caches) OR, if you can run command line scripts, "php admin/cli/purge_caches.php" (which I run a lot during development).
If it's just a change in the code itself (which doesn't require a change to the database structure or the caches), then just save the changes (assuming you are working with a local server, if it is a remote server you'll need to upload them as well) and refresh the page - the changes should appear immediately.
