I need to replace Rainbird rotary sprinkler heads (R50). The sprinkler
guy said I can do it myself - just unscrew the old one and screw in the
new one.

Do I need to dig around it to the pipe to do it? If not, how can I
make sure when I remove the old one that dirt does not fall into the
hole?

Please, help me with any tips on how to do this.

Thanks!

Steve