Changing a menu link used in the sitemap module but not in main nav
{{ something.content }} is a render array, you can't do a string replace on an array. But you can store the computed string output of the render array, and do a string replace on the rendered output:

{# compute the output of the render array and store it #}
{% set rendered_html_string = item.content|render %}

{# do the replacement #}
{% set modified_string = rendered_html_string|replace('seachtext', 'replacetext') %}

{# print the modified string #}
{{ modified_html_string }}