ui <- bslib::page_sidebar(
    sidebar=bslib::sidebar(
        open="always",
        position="right",
        shiny::selectInput(
            inputId="acc_title", 
            label="Accordion Panel's title",
            choices=c("Title 1", "Title 2"),
            selected="Title 1",
            multiple=FALSE
        )
    ),
    bslib::card(
        "Some content",
        bslib::card_footer(
            bslib::accordion(
                id="acc",
                open=FALSE,
                multiple=FALSE,
                bslib::accordion_panel(
					value="acc_panel", # not "id"
					"Title 1",
					"Accordion's content."
				)
            )
        )
    )
)
server = function(input, output, session) {
    shiny::observeEvent(input$acc_title, {
        bslib::accordion_panel_update( # <-- crucial part
            id="acc", 
            target="acc_panel",
            title = input$acc_title
        )
    })
}
shiny::runApp(shinyApp(ui, server))