Replacing tags with another tag in the text using C# console applicati

Question

code to replace tags with another tag in the text

1) multiple &nbsp tag ,<p style="text-align:start"> tag should be replaced with blank space 

2)<h1>...<h6> tag should be replaced with strong tag

3) <ol start="4">$,<ol start="NaN"> tag should be replaced with "$" for ordered list and " | " for unordered list