remove specific text from certain cells, then multiply the numerical value left by 0.001

Want to keep an eye on the accuracy of my ISP's usage meter
The ISP provides me with daily info expressed in GB's

I am watching usage using pfSense vnstat2, this gives me a table of readings i can cut n' paste into excel
The problem is that those readings are mixed number/data e.g.
12 June "12.1 GB"
13 June "345 MB"
14 June "256 MB"
15 June "34.0 GB"

etc

So I can remove the GB and associated space before it with replace function, replacing with nothing
But is there any way to have a function or a macro to find all cells with " MB" in them, remove the " MB", then multiply by 0.001 to convert to GB's?

The other less accurate option would be to replace all such cells with 0.0005, approximating to their GB value, or just replacing with "0", as these small values have little influence on accuracy of data meters

But I was hoping to automate this as above, both as a learning exercise and to get fully accurate

Thanks
Jeff

Sorry, I should have been more specific
Thanks fro quick replies

Date is in a column on its own - thats fine
Next columns have similar usage data - there is a number value, a space and then eith MB or GB
So each column I am interested in look like:
12.1 GB
345 MB
256 MB
34.0 GB

etc

There are more than one relevant column of data, as I have some of internet going thru VPN, some out on ISP Ip, in the end all thru ISP

Thanks
Jeff

A correct version of this would look like:

12.1 GB becomes 12.1
345 MB becomes 0.345
256 MB becomes 0.256
34.0 GB becomes 34.0