Yes, most Basics had peek and poke commands with which you could read and write specific memory locations. For example - parentheses may or may net be needed, depending on the Basic implementation:

    X = PEEK( 123 )

would read the byte at memory location 123 and store its value in X. Then

   POKE( 123, 42 )

would change the byte at 123 to be 42.

But these didn't normally have so much to do with patching executables to add/change functionality.