To edit the word 'members' forum wide, you'll want to go to the phrase manager.
Go to search in phrases
Search text -> members
Phrase type -> global
all else default
You get 4 results, click edit beside the one that says 'members', and enter any text you want as the translation. In your case enter 'Total Registered Users', don't include the semicolon.
If you want to change the specific instance of the word in site statistics, go to the template manager.
Click your style > Forum Display Templates > FORUMHOME
Locate '$vbphrase[members]' in the template, and change it to 'Total Registered Users'.
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