Note that the human exome size is ~30Mb. So you can take the number of somatic mutations in a given tumor sample an divide that by 30 to obtain the Mut/Mb value (normally > 4-6 is considered 'hyper mutation')
Or maybe you can try this tool
You can start by this paper : https://genomemedicine.biomedcentral.com/articles/10.1186/s13073-017-0424-2
In the method part :
TMB was defined as the number of somatic, coding, base substitution, and indel mutations per megabase of genome examined.