I provided some notes on upgrading 2.0.17 to 2.3.3b. And promised notes on 2.3.3b to 2.4.5, which I'm finally now linking to, stale as they are (written about a year ago - sigh).
There are a few differences from 2.4.5 to 2.5.1a, but this could get you to a good working solution to base a 2.5.1a final migration. I know a number of modules have had to be updated for 2.5.x from 2.4.x.
My info above is not like the detailed instructions written up for installing and configuring the software. It's more a set of notes and you will have to accommodate for those issues (or not) depending on your installation.
Hope that helps.
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