How to change datum settings on Legend eTrex

Try an Old Garmin "Mapsource" version and maps data with very low quality. I read somewhere that new versions of mapsource can't load map to Legend types

You need to make sure your GPS unit and Google maps are using the same map datum.
In your user manual:
http://static.garmincdn.com/pumac/eTrexLegend_OwnersManual.pdf
Page 44, Setup Options, Units, Position Format: I suggest setting to:
hddd.ddddd as this is the digital format and easier to read

In Google Maps, right click - Drop Latitude/Longitude marker and this should show the co-ordinates in digital degrees. Alternatively, you can hit the SEND button in GoogleMaps, and it will send the marked location to the GPS via the Garmin website. You may have to install the Garmin plug-in into your browser first, but it's well worth it.

If this solves your problem, please come back and rate our solution, thank you.

Consistent errors are usually projection or datum related. If you are taking coordinates collected in one projection and displaying them in your GPS in another, you the points will offset. I'm not talking about the decimal degrees 00.000n 000.0000w vs 00 00 00N 00 00 00W. I'm talking about the geographic model to represent the surface of the earth. Google Earth generally uses a geographic projection with WGS 1984 or WGS84 Datum. Make sure you are using the same settings on your GPS.

No you can just load the topo and have the etrex directed to the card that you have installed and deactivate the pre-installed map

Your MicroSD card may be full just from the basic maps. Try larger card for additional maps. The vendors supply the minimum to keep the cost low and then force the customer to upgrade the resources at their own expense.

check with hot key F7

The preloaded maps are VERY generic, mostly major highways. They are not useful for geocaching.

The good news is that you can download free topo maps created by a very generous Canadian soul, Dale Atkin. The maps are free but donations are accepted. He is also working on US topo maps. I've used Ibycus and Garmin topo maps and prefer the Ibycus version.
If this solves your situation, please come back and rate our answer. Thankyou.

You can purchase the Bluechart Marine maps for your area online at Garmin.com. They will sell you an unlock code for a specific area.

You shouldn't have to connect to a computer. I don't have an OS map in front of me but I'm guessing they use UTM with a WGS 84 datum (if the map was produced after 1984 that is). Got to the Setup page then select Units. Change Position Frmt to UTM/UPS and Map Datum to WGS 84.

Have you checked if you have selected the correct Datum for your location, usually is WGS84